You have to look at the business model of the exchange. They way you describe it is how it SHOULD work. The exchange makes money directly from you through trans

You have to look at the business model of the exchange. They way you describe it is how it SHOULD work. The exchange makes money directly from you through transaction fees or just account fees. They would not need to "invest" your crypto in anything, because they have other ways to make money. This is (I think) the way Binance and Coinbase operate. But a lot of these exchanges have attracted customers by offering interest (rather than charging a fee) and/or free transactions. But then how can the exchange make money and keep the lights on? Well they have to "invest" the customer's money. Then the investments go bad and it all blows up.