Here is one of the most recent examples of why storing all your cryptocurrencies in an exchange is not a good idea for your money. I think we all know by now what had happened to QuadrigaCX and how it had devastated a lot of investors from their exchange. It was not due to a hack nor a system malfuction but the simple reason of the owner dying whom they stated is the only one who have access to their cold wallets. Not unless you are into day trading it's really a good practice sending the cryptocurrencies you own to wallets you have full control with, there's really no reason why you need to hodl those coins on an exchange for too long when you can create crypto portfolios on third party apps or even in Excel/Spreadsheet.