Why isn't there a premium, ad-free Google Search (or Facebook, or Instagram)? Because the most valuable customers (with the most money) self-select out of seeing ads. It would collapse the 2-sided market and create a race to the bottom. There is a dollar amount of advertising revenue per customer, but as John Wannamaker said - "Half the money I spend on advertising is wasted, but I don't know which half".

If the AI companies made their pricing "pay as you go" without quotas, a few insane zealots (power users) would occupy all the capacity and choke everyone else out. Regardless of the cost, the AI providers would lose the ubiquity they currently enjoy, and become a niche tool for rich tech people. They would rather be a mile wide and an inch deep, doing a worse job serving millions of users, because there's a better scaling narrative for legislating and fundraising that way. Like the advertisers there are intolerable indirect effects of letting valuable "power users" spend more money to get a better experience.

Because sometimes you can make more money by reducing costs and making something shittier (especially if you do it covertly), compared to increasing prices.

I suspect more customers are lost a lot faster when you increase prices, compared to enshittifying the product. It's also a lot more directly attributable to an action, and thus easier for an executive to be blamed if they choose the former over the latter.