[eru]

Well, they could have everyone swim one after another in the same lane. It's not like there's much interaction between racers, as there is eg in a boxing match.

(Of course, this would totally ruin the sport for spectators.

Following this logic, for any sport with no interaction, everyone could just stay home and run around their own track whenever they feel like and just send in their times to a central record keeper.)

[xxs]

Having a competitor next to you provides a stronger absolute result. It's believed to be a standard natural selection/evolution effect. Even pain tolerances increase.

[usrusr]

Also conditions change over time. Indoor pools are much less affected than e.g. a ski run in inclement weather or ski flying where competitions are basically lotteries in who gets a favorable gust, but subtle changes in e.g. air oxygen content could easily cause performance differences in the range we are talking about here.

[xxs]

>ski flying where competitions are basically lotteries in who gets a favorable gust

That has always bothered me that there are some calculations/arbitrary numbers based on the wind... to a point the length of the ramp gets adjusted. I recall when the V shape was introduced, initially it was shunned by the judges for bad style (negative points), regardless it was obviously aerodynamically better. However, swimming would also be affected as the water temperature won't be constant either.

[eru]

Many swimming competitions are indoors, so water temperature can be adjusted to a standard easily for them.

(But you are right about outdoor pools. They are much more of a hassle to adjust the temperature for.)