Yeah, but when I look for collaborators for a game jam, I'm not going to rifle through ten different servers and hundreds or thousands of "job offers". I'm gonna post in the "Searching for a team" section on the jam's discord and forum to find people who are already committed to doing the jam. Most jams are so small that this is absolutely fine.
Having "Stepstone for Game Jams" just seems... overkill, y'know? And most people will just keep using the platforms they are already on for a bunch of other stuff, rather than sign up on a new website for one very specific task.