Sure, they could. Would that be any different from how a bank could steal funds from a traditional deposit account?
By making a bank the custodian of your crypto wallet, you're placing your trust in them and should have similar legal recourse you would have had with a fiat deposit.
I am not sure if you are objecting. Definitely you need to trust your bank if you are going store your crypto with them. I just do not see any large traditional bank stealing their customers' crypto and hoping to get away with it.
As far as I know all the cases of stolen crypto have been newly founded companies with their only business being your crypto. That is quite unlike the other kind of bank.
By making a bank the custodian of your crypto wallet, you're placing your trust in them and should have similar legal recourse you would have had with a fiat deposit.