Why aren't there multiplayer player runs for city trial (idk anything about other modes) that would be cool. Is there a reason it isn't a thing?

Typically, I would consider that categories are worth if/when someone is actively running them. If someone had a run waiting to submit because the category isn't created yet is a lot better than a hypothetical.

me and my friends wanted to do one so maybe it'll happen

It should be created. Me and my friends would submit a run too.