They could get almost perfect fairness in any kind of Olypmic-style race by just letting competitors all use exactly the same lane, one after another.

In earlier times, measuring equipment wasn't accurate enough, so races had to be done in parallel with people starting at the same time. Today, that's no longer necessary. In fact, people racing one after another is exactly how we hand out world records.

Of course, the Olympics and other events like them aren't there to find the best athletes in some absolute sense; these events are there to entertain spectators. Otherwise, swimmers could just do time trials at home and mail in times.

And spectators like people racing each other at the same time.