
You don't need to. If it's mathematically harder to score, it's mathematically harder to overcome a deficit. Simple as that.
You grew up with it, so you like it. I'm giving you an outsider's perspective, and aggregates just feel cheap to me. Again, they aren't used in most major sports, US sports or otherwise. Tennis, for example, 6-0, 6-0, 5-7, 5-7, 5-7 = 27-21 aggregate, but player B wins the match because he won more sets.
But again, I suppose it's fine for soccer because it promotes aggressiveness and offense, which soccer needs more of.