The people whose job it is to measure this have explained it and it's linked above - because it changes employers' incentive to negotiate and makes them more reluctant to give any raises because those raises would be visible and trigger more raises being requested. This can reduce inequality but bring the averages down (some might argue that's an acceptable tradeoff).

There's a difference between coworkers talking to each other and the system being set up in a transparent way where everyone can see everyone else's salaries, these two aren't the same thing.

that is what bonuses are for, and can be tied to individual performance

Generally that's basically how socialism works: if you make things more equal, they tend to be smaller on average. Sometimes that's a reasonable trade, but it's worth acknowledging.