Yeah, as pointed out by TheUltraElite above, this is exactly how scams and Ponzis operate. If they just stole the money of the first couple of people to deposit, they would make a tiny amount and those users would warn everyone else that it is a scam. So instead they pay out as advertised, enticing users to both deposit larger and larger amounts whilst also referring their friends and advertising this great service. Once they have much larger amounts being deposited, that's when they disappear.

Why would a random crypto start up be able to offer guaranteed returns far higher than what the world's biggest banks and financial institutions can offer?