The idea was for the inf to travel as fast as they can and be as close to dry from the Landing craft as possible. Leaving from the back puts them in deep water(sand bars) and slows down the push to get off the beach. The amphibious tracked transports had marines hopping off the side of the boat in the pacific but either way you go about it you get to be a juicy target for a few minutes as those things slowly pull up to the beach. You do have a good point about troop vulnerability and one really interesting thing that had to have influenced allies on beach landings was the 1915 Gallipoli Campaign. TL;DR Front unloading was for speed.