Because why would they? Isn't "being your own bank" one of the selling points of bitcoin and most cryptocurrencies? Opening a cryptocurrency bank defeats the purpose. If you want to hand over your control over your bitcoin to an authority, then send them over to exchanges.
You have a point but isn't okay if we also do have our own crypto bank wherein we can easily transact bitcoin anytime, where in we can easily transact and cash out money anytime we want, so better to have it so that cryptocurrency will become more popular.