That's not to say that everyone does it, but a high amount of the VRchat population are "mirror dwellers" (especially the day 1/early adopters). Further, when you look at world populations and see HUNDREDS of people in that world, but only a few instances that make up maybe 100 of 300, that's because a lot of people have developed cliques where they go into private/friends only worlds and sit in front of mirrors. You can still find people who enjoy exploring but if you look at a lot of the popular worlds they will all have mirrors, and there will always be people standing in front of them socializing. It's just a fad of sorts idrk. Personally I don't get it but I've heard from others that it reduces their social anxiety even more, basically removing the eye-eye face-face socialization (granted it IS in VR, still). For me, it takes me OUT of the interaction and I dislike that the interactions feel less genuine, to each their own