I have seen the same thing in a local slack I frequent. Not weekly, but once a month or so.

It doesn't last long, but it sure is annoying. Sometimes they even join and then spam DM rather than post in a public channel.

It must pay off often enough to make it it worth it, but I can't imagine hiring someone I found through a spam message.

There's no way this pays off. It's probably some enterprising scammer that's selling his bots to individuals (aka suckers) to spam with.