You get rewarded by qualifying. If you perform better in the heat that doesn't mean you should get an advantage in the race. Random allocation would be better.
Why do you believe this should be the case? Keep in mind the majority opinion of most forms of racing and other competitions is to reward competitors for their qualifying or heat performances by assigning the most valuable starting positions to the top qualifiers. This is generally considered more fair than randomizing the final’s lane assignment (or grid order, etc) because it reduces the chance of BS in the finals itself and avoids perverse incentives in the qualification rounds.
My reservation with swimming is that if you were randomly assigned to a middle or side lane in the qualification stages, your advantage or disadvantage is compounded as the competition proceeds. It's not clear to me how lane assignments are conducted for heats; perhaps they use a seeding system.
https://www.nbcolympics.com/news/swimming-101-olympic-compet...
If you wanted a fair system, there would be no qualifiers and no simultaneous racing. You would just have everyone compete at home and mail in times whenever they have a new record. Like eg speedrunning for computer games is done today, or like any world records for fastest time are done.
Track conditions can change throughout qualifying so that the later qualifiers might have a slightly faster or slower track.