Having widely available phone support, even with fancy automation, increases the price of the product you are selling, as even at minimum wage, a user calling in and fumbling their way through a poorly-worded and probably-not-even-legitimate complaint simply costs too much.
A lot of companies, even (if not especially) these very large ones that rely on their serious worldwide volume to turn a profit, are operating on margins lower than the food marts attached to gas stations on sticks of cheap gum, and yet customers often expect the level of service you get from an Apple Store.
As an example of this, I will cite my business: I sell software, mostly for $1, and the margin I keep after transaction fees and sales taxes, is on the order of pennies (not even dimes) per purchase; and in some jurisdictions, due to various pricing/taxation policies I should change, is negative.
If a customer asks me a support question that takes 10 minutes just to listen to (that's $1 right there), it's "game over": unless they go on a massive buying spree to get maybe a hundred programs from my store (unlikely), I have now permanently lost money on their account.
That is what tends to happen over the phone. People don't think of ten minutes as very long. Hell, I know I've done it myself: I call some company, and I end up nervous on the spot telling some rambling five minute-long story that only at the end remembers there was a point, and then have to clarify again.
For a concrete instance, I was recently left a voicemail by someone calling me because they purchased a phone from me off eBay and had some questions about the software on it... I don't sell phones on eBay: someone on eBay simply preinstalled a program of mine (this is quite common, btw). Just that part of the subsequent conversation with her (I decided to call her back, which I rarely do) cost $1.
The companies you then mention are textbook examples of ones whose scope customers don't understand: an ISP gets calls from customers saying that their Internet access is offline when really Facebook login is broken, or asking questions about email scams they recieved or tech support on their browser.
The result is that yes: a sane company (not mine: my personal cell phone number is on my website, not that it will help you, or even my friends, get through to me any ore) will make their phone number difficult to find in a way that only smarter people will be able to find it, as a filter on the kind of calls they get.
You should probably actually consider this a feature, not a bug: this means that you, someone "tech saavy" enough to find the number--or to break out of the automated menu that otherwise does nothing but say "did you turn it on and off again" for a half-hour--has a chance in hell of getting actual service on the other-side.
Otherwise, you end up in situations where, as 99% of issues aren't even related to the product, and where it takes forever to listen to and understand the customer's complaint, the only thing you can probably do is get a refund, as sending a full refund is often cheaper than the time spent listening to the complaint.
That's actually pretty much Amazon's phone service: you can call them, and an automated system tells you how to ship your product back and get a refund. Most of the time, though, a refund is nice, but what you want is help: you want the service or product to work, but to make that happen, you need tons of cheaper hurdles to jump before "person on phone".
(By the way, even if you never offer a person, phones are expensive: a toll-free phone number runs you at least 1c/m, and could run you upwards of 2c/min; a good estimate is $1/hr. This puts a hard cap on how long a phone call can go before you start losing money on that customer: the best hurdles start before the phone call.)
The only way any of this is then workable at all is if no one ever needs support, so the few with issues are subsidized by the many who don't; but if you have things like "I forgot my password or username" on the list of things people might call about, almost everyone will call you eventually: every low-margin product needs these hurdles.
Again, I've even done that: I once called eBay from a party (Google I/O; I'm pretty lame: that's the most happening party I can tolerate, and I sat in the corner wearing earplugs), such that the person couldn't quite hear me, as I had a concern regarding the eBay login system: result being that I never had an account (apparently). (In my defense, it was time sensitive to me as I believe I was investigating a case of "phone bought of eBay" again with someone who was angry.)
Support is part of the cost of sales and needs to be factored in. So if you get a lot of support calls, you need to either raise the price to accommodate or you need to eliminate the need for support somehow. Either way, making the support experience worse for the customer is a terrible support cost cutting tactic.
In fact, it is actually quite often that people don't want to pay that, even if they can: many of my friends do not actually use customer service on their computers and they know enough not to call our ISP about stupid issues that have nothing to do with them. They therefore tend to optimize their purchases of these kinds of products based on other factors, like price, and if someone says "Verizon has terrible customer support" they don't care if it saves them money they can spend on other things.
What you are seriously saying is that the price of gum at the gas station should be higher so that the gas station attendant can have the time to hear every single complaint you might have about it: I'm sorry, but most people just want a cheap pack of gum. It is entirely market-dependent whether people are willing to shell out more money for better support. Would it be cool if everyone everywhere had great support? Maybe, but it certainly wouldn't be efficient, and people would have to buy a lot less.
As a sad logical conclusion of this, my business model simply doesn't work if you have to provide world-class support. You cay say it is part of "sales", but honestly, in the end, you paid $1 for something: maybe all you need right now is a refund. People think it could work as a business, though, because they see Apple doing just that, but Apple makes their money on hardware with multi-hundred dollar margins, and then breaks even selling apps for those devices only because it is an interesting way for them to sell more hardware.
Yet, people want businesses like mine to exist. People go to thrift stores, they buy stuff at $1 stores, they purposely get Dell laptops (and wait until they are on sale with a special offer code), and PriceLine.com finally slumped, but it did so slower than the highly price-competitive airline industry that people complain about bitterly. Some people just don't feel that support is something they are willing to pay for: it is a form of insurance against the future that doesn't harm anyone else if you choose not to buy it, and so often you just don't and live with the consequences.
Here's an idea for company in this kind of situation: move all call center staff to phone support. And then market the hell out of this fact. Like, "we the company X respect you as a customer, so we moved all our call-center staff to support. We won't call you with useless offers, and you can always call us if you have a problem." It would definitely help getting new customers, as well as keeping the old ones satisfied.
You see this in computers (Apple only targets high-end customers for a reason, and you pay for all of those happy people everywhere), you see this in airline tickets (nothing makes me angrier than someone who complains about the food on a flight they optimized down to the penny using a service like PriceLine.com), you see this in food (people get angry that the food they eat is unhealthy, but don't take this into account as part of the price)... you just see this everywhere: people optimize on the one thing that is easy to optimize in the moment, and that is price.