A question about exchanges and wallets.

I have multiple exchanges I use to buy different types of crypto. I only have 2 wallets I use currently. Are my coins at a greater risk being on only 2 different wallets? Would it be better if I keep some on exchanges? Would it be better if I opened more wallets and spread them evenly? I’ve always seen people say never to leave your coins on an exchange but what is the reason? Thanks