It's because AI is just software, and like most software, it's shit. Almost a net negative for people. Obviously you can't sell a neutral or net negative product, so you just do what software always does, offset it with ads, i.e. consuming human time instead of money.

If a service has real value people actually want, then it doesn't require ads. If the service needs ads to be viable as a business, that's a strong indicator the product or service isn't very good or useful.