I’ve got a laundry list of reasons, but suffice it to say that pretty much every third party client I’ve ever used has been miles ahead in UX and polish.

One example is that if the API throws any error response and lands you on an “Error” page (post removed, user deleted, etc), the whole UI is stuck there until you refresh the whole page (e.g clicking “back” updates the URL to your previous page, but you’re still seeing the error).