Because then it means they get to care even less. It's still their infrastructure and therefore their van that needs to come out to your house to fix things, but you've added yet another layer onto the process.
They've worked their contracts with the ISPs so that the ISP can't actually do anything in your name, so you still end up on the phone with them, but you're not their customer so you can't mention anything being wrong with the internet as clearly that's your ISP's problem. No, now "the phone line is really scratchy" is your only line of attack and god help you if you've contracted that out to a 3rd party (or worse, your ISP itself!)
Then they've cleverly broken off the van that comes to fix things (BT OpenReach) as a separate company that you can't talk to, so you have to go through BT's call center, which will helpfully tell you that there's no fault, and that it will cost $150 to send a van out if it turns out they can't find anything (d'ya feel lucky, punk?).
Fortunately, the van guys are actually good. And BT's infrastructure is so terrible that there's always at least one fault they can find. Usually about 5 meters further down the line than last time they were had the truck out two weeks ago.
One day somebody will lay some fiber that has no BT-ness associated with it at all. And they'll get everybody's business.
My recent experience trying to get BT Infinity installed...
1. Initially called and asked for BT Infinity and was promptly told it wasn't yet available on our street. I therefore opted for BT Total (ADSL). All installed no problems.
2. Checked the website a month later. Checker tells me that Infinity is available. I call BT, make the appointment and wait.
3. BT engineer turns up and starts installation. He then goes off to the cab and returns quickly telling me that the cab they have me listed on is wrong. I'm registered as using is about 2 miles away. Far away from this cab. BT engineer tells me he will inform "head office" so they can sort it and get Infinity installed.
4. BT make a new appointment and another engineer is sent out who repeats the same steps as above because nobody has told him and it hasn't been fixed. Engineer leaves. No Infinity.
5. I get a SMS message asking me to rebook my appointment and after calling I talk with a operator who is ready to put me through the whole process again until I protest asking to speak with somebody who knows more. I get on the phone to this person who tells me that Infinity is not available where I live. I'm told to check back in a few months but get this...I'll have to rebook the appointment and get a engineer out before I find out if it's actually available.
Counterpoint: Fibre has literally just been made available in my area. I just had Plusnet (an ISP wholly-owned by BT which uses BT fibre) install fibre broadband, and it was done quickly and flawlessly. Arrived when they said they would and it took 5 minutes. Speeds decent too (50 Mbps down / 20 Mbps up, though I was told it might take a week to reach full speed).
However, simple fix that for - don't use BT as an ISP - I don't know where you are but we have about 20 to choose from.