[Blue Jays]

I am with you and totally see what you are saying.
If they had a way to "back-in" an airplane and put the jetway to the door at the rear of the fuselage, I'm sure they would.
Maybe it has something to do with wing clearance near the terminals, other aircraft, along with the jetway itself.

[Behind the Blue Wall]

No, I would say the opposite if the door was at the rear. It seems like the fastest way to board a plane would be to board those who have to travel the farthest to their seats first and then move back from there, as that then minimizes how many people have to wait in the aisle while someone else puts their stuff in the overhead bin, lets someone into the middle and window seats, etc.