This has become an established convention that goes unquestioned these days but it dates back to the early days of aviation.
In times past an aircraft would taxi close to the terminal building in order to load and unload passengers. Since by convention the pilot sits in the left-hand seat of the aircraft it’s much easier for him or her to judge wing clearance if they approach the terminal on the left side of the aircraft.
Left side doors therefore made it more convenient for passengers to transit between the aircraft and the terminal, as well as safer than having to circle around the aircraft.
An even more basic reason for left sided entry/exit dates back to the very early days of nautical travel. Before centre-mounted tillers, ships were steered by a steering oar mounted on the right side of the ship.