Back then when bitcoin is still in its infancy, there is not much to do with bitcoin in your wallet apart from gifting each other some coins (bitcoin faucet was pretty generous back then). So people on bitcointalk forum often openly running some ponzi scheme and a lot of people would join just for the heck of it. There was nothing shady about it as the organizer would openly admit its a ponzi scheme (something along "hey, I'm running a new ponzi scheme here. If you're interested here is the url").

No idea why you're getting downvoted. It's completely true and I participated in a few. The excitement was that you knew it could blow up at any time so it was like gambling.

When you got your profits you often sent it back to the scheme. It was a classic chicken race where you wanted to pull out just before it crashed.