But it really doesn't matter. Most flights will finished boarding well before the departure time, and many will arrive late as they always do. The plane can't leave until the departure time, so unless there's a direct cost benefit, why bother?

They load so early because they have to guarantee that everything can happen in time. If you can reduce the time it takes to board, you can board passengers closer to the actual departure time. This allows you to squeeze flights closer together, which saves a ton of money.