ignoring topics of meta compositions and such, playing mirror comps is just easier for most people.

If you have two teams playing completely different comps, and one of those teams is losing, it can be difficult to tell why things aren't working, whether or not your comp is the problem, etc.

But if your team is playing the same comp as the enemy team, then you know your comp is probably not going to be the problem, because both teams have all the same tools, the same general playstyle, the same various win conditions. Both teams are on level playingfield, and all your team has to do is play that comp better.

This isn't nearly the only reason for prevalence of mirror comps, but it is a part of it.