Do they?
What is your evidence for this?
Also, if we assume they do, well ... their BTC is now potentially subject to seizure, hacking, fees, bankrupt exchanges ... well, their choice.
Darwinism at work as far as I'm concerned.
Not you keys, not your coins.
There is about 250k BTC on ETH as WBTC, the largest secondary pool in escrow. Ren has about 18K BTC in escrow. Lightning network is about 3k BTC.
Bitcoin maxis literally go insane telling people to take their coins off exchanges. It's part of the dogma.