Why should Bitcoin holders worry about something that has nothing to do with Bitcoin? Exchanges aren't called exchanges for nothing, they're places where you trade your assets, not where you permanently store them.
Because they did not understand purposes of exchanges or know about that but just used exchanges with their risky ways.
In addition, I'd like to emphasize that risk starts at a time you fund your exchange account, and it unlikely exists if you store your fund on exchanges permanently (I don't think people do that) or a long time.
Short time, long time or permanent storage of your coins on exchanges is risky. Clearly if people store funds on exchanges a long time and don't mind to regularly check that exchange operational status, their accounts, balances, it brings more risk.
Thoughts on secure storage of bitcoins and other crypto assets.The number one rule to storing crypto assets is this: if you dont hold the private keys, you dont actually own the assets. As a result, I do not recommend ever storing crypto assets with custodial online services such as a web wallet or an exchange.