>Among other things, because of a dead-simple user experience that lets even grandpa post content after a few clicks. Federation, if it adds any level of complexity past the grandpa UX, would defeat the goal.

You could have stopped there, that's pretty much the only reason. But that has little to do with federation though. And yeah the ugliness and bad UX problems of free software - software that's freely unusable - have been known for literal decades.