But, those exchanges are only trying their best to improve their security and deliver a great service to their customer, in order to become the best in this field. Okay for their confidence, they can also store funds which they can afford to lose easily.
The exchanges you listed are all those that have been around for many years, and they also have a certain reputation in the market, I love coinbase and binance, the top 2 exchanges right now. But in this market, anything can happen, they were safe in the past and so far, they are safe but that doesn't mean they won't crash in the future. Mtgox was the biggest exchange in 2014, no one thought it would crash until it did.
Everything has risks, especially centralized projects, so it's better to be careful, we still use them but don't rely on them too much or trust them completely.