Why are certain questions deleted off this site?

For instance, I saw a question asked titled ‘Can birds fly?’. Now I know, my first opinion was this guy is an idiot, but trying to give advice I said ‘ well, most can’. I then decided I’ll find a list of birds that can’t fly to elaborate on my previous advice and be more helpful. When I did, and went to reply, I noticed that the question had been deleted.

Now, I know, it may have been a stupid question in the eyes of some people, and may have even been a joke question written by the person who asked it, but it was still a question, and it did not breach the ‘terms of use’.

So why was it deleted? and who was the person who decided that what may have been an honest question by someone who may be intellectually disabled was unworthy of being on this site?

I see questions like: ‘What should I wear to the mall?’, ‘How can I make my penis bigger?’, ‘How can I get bigger boobs?’ and the classic ‘I am 5’8” and weigh 20 pounds, how do I lose weight fast?’, which I think are completely stupid, and they stay on the site, so why does ‘can birds fly?’ get deleted?

I mean it isn’t that stupid, It wasn’t like he was asking ‘Do fish drown in water?’ or ‘What is the best way to dispose of my neighbours corpse?’ I was going to elaborate on it because he may have made a simple typing error and meant to ask ‘which birds can fly?’ or ‘which birds can’t fly?’

So at the end of the day, and so I am asking a valid question that shouldn’t be deleted off this site…

Why are some seemingly stupid questions deleted off this site while other seemingly stupid questions are not deleted off this site?