Why don't more software not use it?
Not only do they have control over your distribution channels, but the rules they impose change over time, forcing people into whatever Apple wants. There was a blog post on HN a while back where an App update was rejected because they failed to hype up the new iPad Pro's or another post where an App was rejected because (by sheer chance) somewhere the description referred to "an unreleased product".
The only real benefit is that it might bring your software in front of a lot of eyes, which, as a big player, you don't really need that much.
Just missing a Cloud (Dropbox/iCloud) HomeBrew configuration and I would be set for the essentials.
As parent said big stuff don't need to exposure, and anyway, as a developer most of those stuff wouldn't make it to the appstore anyway.
edit: also forgot licensing. I rather buy it on the AppStore so I'm also avoiding having to fish the license email and/or create an account with whatever utils company.
edit: For browsers, may be related to not using appropriate webkit framework. see comment https://news.ycombinator.com/item?id=18399518
(That's why you're downvoted, probably)
I will certainly agree that their stuff is gonna need a lot of work before it can live in nice tight little packages. It sprawls everywhere, it needs places to put plugins and their files, changing this is a big enough task that it probably qualifies as a major feature for a new release and I wouldn’t be surprised to see it be unstable around some edge cases for months
Not that there isn't crap on the iOS App Store, but I don't think it's controversial to suggest that the Good to Bad App ratio is better there than on the web as a whole.
5.2.5: Apple Products: Don’t create an app that appears confusingly similar to an existing Apple product, interface (e.g. Finder), app (such as the App Store, iTunes Store, or Messages) or advertising theme.
The point still stands for FFx though...
And once you're off the App Store, users won't be nagged to update your application, so you're responsible for getting your users to update all by yourself. At which point some form of over-the-air update is more ergonomic than making users hit your download page to pull in the latest version.
By comparison, Ableton, Adobe, etc, aren't having any trouble shifting units as it is, they have more than enough labour power to run their own infra, and they don't stand to gain anything by handing 30% of their revenue to Apple.
Perhaps the question should be flipped, what would these companies gain from distributing via the app store?
At the same time, you can also put your app in the AppStore and provide a non-AppStore version on a website.
My Mac complains that I need to log in in order to update apps that I never use such as Pages.
If you sell software for businesses, it's pretty painful to actually buy software with their volume programs.
Both Slack and Remote Desktop are free apps that access non-free resources on the back end.
A company like Abobe has high value software and large customers who will not accept a single source fulfillment vendor for software. A school district, for example, is usually required to get competitive bids on a state or other contract from multiple resellers. Usually companies drive revenue by bundling, so putting product A with Apple and product B with the channel is complex and doesn't add value, particularly when Apple wants 4-6x more dollars to do that fulfillment.
None of the precludes simply selling the very same code product (sans bundle) on the App Store at a price that's higher than your bid price. Doing so, increases sales (perhaps marginally, but more is more, and the opportunity cost is trivial), and it makes your bid price appear even lower.
The reason why Adobe doesn't bother is because they're a household name. People that want Photoshop, will just google it and go to their page and download it directly. The 30% acquisition cost simply isn't worth it when you've already cornered the market.
But Adobe is an outlier. They own a verb. Most people aren't in that position.
Why many apps don't use the Mac App Store:
1. It's primarily because software distribution on the Mac had different mechanisms maintained by developers for decades (or from before the iOS App Store and the Mac App Store came to be). Many old time customers are used to buying direct from developers and having a way to have a direct contact with the developer. For example, AgileBits (maker of 1Password) had to go back to retaining its own sale/distribution mechanisms after declaring that it's going MAS only because of long standing customer backlash (and probably acknowledging that it was anyway forced to maintain this for its Windows app at that time).
2. The App Store on iOS is deficient in many ways, when you consider that it doesn't allow (easily implemented) trials, upgrades, etc., that many users from the PC/Mac computers side are used to. The Mac App Store is even worse than that. Read this mess about free trials on the App Store (iOS) from a few months ago (June 2018), written by developer Daniel Jalkut of MarsEdit fame. 
3. The sandboxing restrictions that just don't work for several apps, means it's too much of a hassle to change (or cripple) one's app just so it can be distributed by/through Apple.
4. The 30% cut that Apple takes is probably a lot higher for developers who already have had a system in place for a long time. But some newer developers have moved to the Mac App Store as the only means of distribution (Pixelmator is a popular and notable example that went MAS only and has stayed that way).
5. For developers who care more about customer relations, Apple's country restricted stores and the lack of portability mean that a user who moves from one country to another would usually have to go through some confusing ways of updating apps bought from a previous country. As for subscriptions and in-app purchases, those cannot even be transferred, AFAIK. Even now, subscriptions cannot even be canceled before they expire (you'd have to contact Apple for that and try to work it out). The app stores are completely Apple owned and controlled, but yet they don't have any easy solution for people who travel or move to another country (or to other countries) either temporarily or permanently.
I don't really need Apple to help with marketing the app, so paying 30% on the Mac App store is way too much for me. On iOS, it is necessary because there is no alternative. Also, my simulation tool has an enterprise subscription, which means Apple can say yes or no and meddle with my business model. Too many restrictions!
Now Apple is saying I have to get my app notarized for Mojave. I don't quite understand why I have to do that in addition to the $99/year I pay for the Mac code sign. Bottom line is that I'm not making a killing by any means and Apple is hurting me by adding all the technical restrictions and financial constraints.