As someone mentioned, there is no reason for a rational user to risk dealing with a fractional reserve bank with Bitcoin, unlike with gold. All it takes is one bank publishing 100% transparent blockchain data about accounts in custody. Then who is going to put up with fractional reserve?
Most humans are not rational users, to begin with. And even those who can be considered as rational on the whole are not rational all the time. If it were so, why would we in the first place still have fractional reserve banking now when gold is obsolete?