People leave money on exchanges because the alternatives are worse. They're almost as dangerous (if not more so): how many people have lost millions or billions due to malware, forgotten passwords, failed hard drives, death of the owner and inability to transfer that wealth to their heirs, etc.?

All of these problems are solvable with ludicrous levels of complication and confusion, and the average person doesn't even realize they need to take these sorts of measures until they've irrecoverably lost everything.