I think the rule was created because underwater racing is not that interesting to watch for spectators and more difficult to officiate from the surface. Maybe all we need is a bunch of GoPros stuck around the pool and we can see a new race category?
A swim coach told me that in 1950s people used to do the first lap of breaststroke underwater but people kept passing out. It wasn't safe for youth sports.
The real reason is safety. People have known about the efficiency of swimming under water for the longest time, but competitors would pass out or even die when they held their breath too long. So the rules changed to force people to the surface.
