Planes have 4 doors, two front, and 2 aft, and yet the last time you boarded a plane, you probably only entered through 1 door, near the front and on the left side of the plane. It seems like an industry that needs to keep planes in the air would use those other doors and stuff people in faster right? You would be very intuitive but also wrong. There are a variety of reasons that you only board on the left side of the plane and only through one door and it has nothing to do with airlines or efficiency, it has to do completely with airports.
In this video, Bright Sun gives us some interesting aviation trivia on why we board planes the way that we do.