It's to keep design space open and to minimize developer work.

Let's say we decide to keep an overperforming gun. It does all the things. It has all the ammo, all the damage, all fire rate, all the reload speed. Now, all future weapons have to be made with that as a consideration. Why would players choose this new weapon, when there's the old overperformer? The design space is being controlled and minimized by the overperformer. Players will complain if new weapons aren't on the level of the overperformer.

Now, let's say we have ten weapons with one clear overperformer. Now, we can either nerf a single weapon to bring it in line with the others, or buff nine weapons to attempt to bring them up to the level of the overperformer. Assuming the balance adjustments of each weapon are the same amount of work, that's 9x the effort. However, if we assume we do this extra work to satisfy players, now we have ten overperforming guns and players find the game too easy, so now we also have to buff enemies to match. However, the game isn't designed to handle these increase in difficulty. Players complain if we just add more health to enemies, so we have to do other things like increase enemy count, but adding more enemies increases performance issues. It's a cascading problem.

I consider nerfs a necessary evil. It's absurd to ask developers to always buff weapons and give them so much work when they could be developing actual additions to the game. Sometimes, a weapon really does need a nerf.