It is stupidity, greed and denial of facts. I know that people with small deposits keep their funds on exchange, because they dont want to spend those few bucks on transactions. They prefer to risk with everything just to save a bit. People tend to think that bad things will happen to everyone, but except them. There was and never be an exchange that wont scam, be hacked or all of a sudden freeze users account. It is just a matter of time. I dont know how many more exchanges must scam people, that will make them keep there funds they use for trading only.
You may consider it stupid, but looking at even the bitcoin developer at the top being able to get hacked, I would say nothing that you could do is safer than keeping it at Binance. I know many people would say "what if binance gets hacked" or something of sorts, or binance being a scam eventually and what not, or even bankrupting, but I think those are smaller chance than me not getting hacked.
I feel like if I store it myself, eventually someone could hack into me, I do not trust myself, whereas Binance is strong and at the top and never failed to provide withdrawal to customers ever. Hence, it is easier to make sure that your money is safe with them.