We need to have an easy way to pay small amounts for a one-time service. A lot of websites offer content that you need only a couple of times in your life. It's worth paying for, but not worth all the hassle of setting up a normal payment.
This leaves ads as the only form of revenue and because ads don't care about the content, this creates a race to the bottom on generating slop.
The advertisers do care about the content. Ad based models ensure that content doesn't piss off advertisers. User payment models ensure that content doesn't piss off the user base, which does sound better, but audience capture can be every bit as bad.
This leaves ads as the only form of revenue and because ads don't care about the content, this creates a race to the bottom on generating slop.