Why is there a deadline for things when casting people will just fill it before that date?

I don’t understand why casters set deadlines (2 or 3 days before where you have to get in by) and then fill the role before that deadline is even up. Does anyone else think that’s way too quick of a turn around, or is this just show biz? I don’t know when I should be filming these self tapes if this is what is going to happen. As I’ve noticed a pattern recently.