Because at that point, they are a bank. Nothing fancier, nothing further, a bank. They aren't going to be a better bank than the bank at which they deposit the real money, so what do they do instead? Just say that they're totally not a bank, not at all, and that's why they don't need to do any of the fancy paperwork or background checks or meet minimum reserve requirements, because those are things that banks are required to do, and they're definitely not a "bank".

So now there's a business where the only way to make money is to break the regulations that exist to stop banks from doing exactly what they're doing. It's in the cryptocurrency environment, where the entire culture is made up of scammers, chumps, and chumps who aspire to be scammers. And in that environment, there's a big pile of money that could be transferred out without any oversight at all.

At that point, it isn't just that people "become greedy", but that every single aspect of the situation is aligned to select greedy people and create a situation that rewards theft.