All sports include arbitrary rules to try to make them more fun for both participants and viewers. Basketball isn't just about seeing who can get a ball through the hoop the most times, car racing isn't just about who can be the first N times around a track etc.
But honestly, it annoys me when things like swimming and car racing bind themselves up with rules that amount to "no, do it the conventional way, not the better way". It should be legit to win by innovation.
g-forces causing harm to drivers: https://what-if.xkcd.com/116/
Smaller events also have to allow the competitors to focus on a smaller range of capabilities so they can actually find competent players. Robot and drone competitions come to mind where the equipment is often standardized so they're only testing skills or programming, not being-the-best-at-everything which would end up as "throwing the most money at it".
The official reason? Safety concerns (from oxygen deprivation) and the fact that people swimming underwater makes for a less spectator friendly sport. There might be some other unofficial reasons, but not sure what they might be. My guess is that the spectator angle is the main reason.