I guess people don't care enough. I've never personally participated on these, even if some exchanges failed in the past on these, what does it matter now, I'm always able to withdraw my crypto from the exchanges I've chosen to use anyway, stressing their servers for no reason of course will clog them up, I think people just didn't care enough about it.
Well its one day later since you have posted this comment and in that time Kucoin has just been hacked for 200 million usd. So this kinda proves my point if not about proof of keys but about how secure your money is on an exchange.
And yes you can say that i don't use Kucoin or it always had a bad reputation. I only use binance or coinbase etc and those are safe. But in reality both of those exchanges have been hacked in the past as well, it is just that they were large enough to be able to cover losses associated with the hacked funds. So can you really just depend on the fact that the hack will not be large enough to effect you or take the exchange down.
So i say again if u want to keep some money for trading on an exchange fair enough, but if you are using that has your primary form of storage for your Bitcoin or other cryptos then you are only asking for trouble.
Go check out the Kucoin telegram channel to see what a shit show that is for users whose funds are now locked.