Savings in exchange are very practical for trading but also very risky.
I don't understand why people keep their coins in exchanges. If it is for trading purposes, then they should immediately withdraw the coins once the trading session is finished. But many people keep their coins in exchanges for long duration, as evident from the Bithumb hack.
As you know, trading is unpredictable, the price could change every time, how the traders could make a profit if they always withdraw their coins? Just use your logic, the only most efficient way is to keep it on the exchange.
Just hoping that there'll be a really good decentralised exchange but let's come back to the reality.
Even if so many people have recommended to not keep your money in an exchange, it's still a hard thing to do because of many factors.
By the way, there's no such thing as "trading session is finished", trading is always taking advantage of the circumstance.