If users deposit money into an exchange and that exchange keeps the user money and don't tempered with it, how will that exchange ever fail? It's like given somebody money to hold for you and when the time is right that you need your money back, they give you exactly how you you gave them and everything settle. As for the exchange, they benefits when you want to withdraw because they charge you more than they asked you for withdrawals and also gains from fees.

The exchanges that we see collapsed most often tempered with customers money and when something goes wrong, they don't know a way out because nobody will help them out with the money, not even a fellow exchange will want to lend them because they don't have money on their own in the first place, it's all customers money and given it out will jeopardize their trust with customers as well. When exchange put greed before anything, they will likely fail.