Seriously, _what_ are they gaining by eliminating access to third-party clients? If they want usage data, they already have all the API calls. If they want more ads, they can change the APIs to inject them.
Reddit wants freedom to arbitrarily change the design of their app and placement of ads, etc. Ads are a huge (primary?) source of revenue for them.
If they are tethered to supporting third party clients, it's harder to make reasonable estimates of how many captive users will see ads or new features.
Reddit could enforce ad presentation in third party clients, but to appease advertisers Reddit has to make guarantees around visibility. It's not enough to check if third parties are calling the correct API, they will actually need to regularly audit all third party clients.
It really isn't worth the time or effort if you can just charge third parties the cost to cover loss of ad views.
Google and social media platforms have shaped the web to be entirely advertisement driven. If they were capable of showing you things you wanted to buy, without the creators paying to be seen, they'd never make any money.
Almost anything you ever want to do, someone else has already done well, but despite that, it's hard to find snippets of code you can include in your projects. It's easier to just write it all yourself. If the usefulness of ChatGPT is an indicator of anything, it should be an indicator of how much is out there that you never get to see. The sad part is realizing that that's intentional.