Because it's a social environment. You don't go purely on appearance usually, you see someone interesting, start chatting, maybe you flirt a bit and if the vibes are right you move forward. If anything it's far less appearance focused than the apps where it's a picture and a bio and not interaction.

As for why not coworkers and classmates and such, it's fine if flirtation is happening. But to a certain degree it's shitting where you eat. People have professional and to a lesser degree academic personas that are less who they really are than their social personas.