Because they might get turned down.
I’ve been asked a couple of times - the answer was no.
I proposed to my husband. Well, not really proposed, but let it be known that I wasn’t going to stick around for another year if the relationship wasn’t going to go in that direction and pretty soon (we’d been dating for 3 years). He got the hint.
I’m quite sure there is a lot of that type of ‘proposing’ that goes on.