It's a quid pro quo thing. They all have to tell the poster that they're beautiful so that then when they post their own photos the poster will be among the people who tells them that they look beautiful. This little circle jerk is important to them and if they don't keep it up, by commenting, then one day they might post a photo and nobody will tell them that they're beautiful. That would force them to confront a reality that they just aren't equipped to handle.