- It's a very niche thing to charge for, and merely charging for something means having to support it, so you can be underwater on support costs alone
- Users on third-party clients are resistant to enshittification
The business model of any Internet platform is to reintermediate: find a transaction that is being done direct-to-consumer, create a platform for that transaction, and get everyone on both ends of the transaction to use your platform yourself. You get people hooked to your platform by shifting your surpluses around, until everyone's hooked and you can skim 30% for yourself. But you can't really do this if a good chunk of your users have third-party clients.
This is usually phrased as "third-party clients don't show ads", but it extends way broader than that. If it was just ads, you could just charge $x.99/mo and make it profitable. But there's plenty of other ways to make money off users that isn't ads. For example, you might want to open a new vertical on your site to attract new creators. Think like Facebook's "pivot to video", how every social network added Stories, or YouTube Shorts. Those sorts of strategic moves are very unlikely to be properly supported by third-party clients, because nobody actually wants Twitter to become Snapchat. So your most valuable power users would be paying you money in order to... become less valuable users!
If social media businesses worked how they said they worked, then yes, this would actually be a good idea. But it isn't. Platform capitalism is entirely a game of butting yourself in to every transaction and extracting a few pennies off the top of everything.