>>11423275babyfaces want to prove they're the best and beat the rest of the field. Heels do it occasionally but it's one of those spots that can't really "work" in kayfabe because no one should want to be in the battle royal if they're the sniveling heel type.
I think there used to be a rule in the NWA rulebook that you had to be in the ring before the next man entered? In the giant "20 men start in the ring" battle royals it obviously isn't an issue.
It strikes me as sort of like "why do people ever tag in and out in a no dq tag match" where there's sort of an answer (only the active competitor can pin the other active competitor) but there's no reason you shouldn't pull out a chair immediately and fully 2v2 the other team.
That's what I can come up with, anyway. My gf is learning about pro wrestling and asks me these sort of questions and it's obviously hard to have "real" answers at times but she's loving it.