Actually i sometimes store my Coins in exchanges but only if i plan to make a trade specially when market is Moving forward or downward .
Those chances are limited and now since i stopped in trading so i chose not store my coins in my wallet and those for Holding is in my Ledger.
That's the only instance where I would agree about keeping coins on exchange. But the story is different especially for those who only use exchange wallets as their primary wallet for storing their crypto as they don't want to take time to setup a wallet - stuffs like saving mnemonics and priv keys.
The exchange hacks we have had in the past are more than enough reasons not to use exchange to store cryptos. The primary purpose of using an exchange is to only do as the name implies - to exchange from one crypto to another.