The fragmentation of streaming services often conflicts with what consumers want to see. Spotify is a centralized service and people love it. That's basically what consumers want, a Spotify for TV. The other issue with all of these streaming services is varied compatibility based on your platform. HBO didn't work with Roku for a year or two over a contract dispute, other streaming services weren't available on all devices. Granted, I haven't run into this issue lately, but in the era of "smart" (eyeroll) TVs and other entertainment devices, you can't really assume that anything "just works" anymore without checking first.
YouTube didn't work on Amazon Fire devices for a long time because Amazon removed competing Google products from Amazon.com. It was really frustrating. The consumer always loses in these cases.
Because the real reason is that people do not want to pay the amount that the sellers are asking, not that they have to visit a different website or app.
I can easily search and watch anything outside of Netflix in the Apple’s TV app right now within seconds assuming I have no problem paying for it. Whether or not it is “fragmented” makes little difference in user experience.
Hulu itself is about the same costs as equivalent cable for me, except I have to buy the hardware (if I need any) and nobody comes out to hook it up. And then there's issues with the stream so very often...
You could subscribe to Netflix/Hulu/Peacock/Disney and still come under a lot of cable bills.