It should already be common sense that you dont store any valueable thing anywhere without being the owner of the "store". Noone would give all of his money to complete strangers and hope that they manage it, so noone should do this with his coins, too.
But it has been happening for a long time and specially day traders are doing this. Recently Binance hot hacked and 7000 BTC was hacked. Aren't all of this are customers BTC, from which most of these are from day traders.