Because the only place where "free" AI is viable is when the model is running on your own computer. It's not sustainable at all, from a business perspective, to let millions of connections absolutely slam a huge AI model 24/7 for free - even with usage limits and throttling.
https://en.wikipedia.org/wiki/Jevons_paradox
Better to write a solid inference engine, tell people to run it themselves, and externalize that cost.