I doubt that, it seems the investment websites used are entirely fake, they probably just use crypto because it's well known and sounds good to many people. They might however just as well promote fake investments in stocks or something else.

Without convenient on/off-ramps this kind of scam would be much harder to operate.

Yeah, the crypto is the hook. They don't have to do any trading once you've moved the money (but they do obviously want to show via UI that you're making bank and that you could/should go bigger)