Because Quora writers know what they’re doing.
Of course, rank beginners don’t know what they’re doing. Partly because Quora has never onboarded well, but it is a reasonable and laudable requirement of the community, that a question with explanation and illustration is more useful (and frankly much more fun to write), than a one-liner I could have found on Google. As indeed McKayla Kennedy said in her answer.
But Quora can’t read writers’ minds. If only there were some sort of probation mechanism, whereby Quora would stop collapsing short answers, once writers had established their bona fides as experienced writers…
… which of course, is exactly what happens. See How long has it been since a bot has collapsed one of your answers for shortness (Sept 2016)?
There’s a lot about Quora UX I think is stupid, but this isn’t one of them.