Because there is a very real sentiment called "Don't shit where you eat." I learned it the hard way in two different workplaces. In the first one, we broke up and it ruined the work environment. In the second, the 'no' was expanded to HR complaints and lawsuits, again ruining the work environment. Knowing someone has (or had) romantic interest in you can be a pain, and it can definitely blow up a working relationship.

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.

Expanding your pool of potential mates increases the chance of finding a mate.

And then there are people like me, who don't have the biggest friend group and it's predominantly male. And my workplace is also highly male dominated. So I don't have any women that I know that I could ask out.