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.
You can cash out your money anytime with exchanges like Coinbase, Gemini, and etc depending on what country you're in. If you want a cryptocurrency bank, then just use your traditional bank instead. I don't see the need to use cryptocurrencies if you prefer having your funds on a centralized bank.
This is an interesting idea. And they are already working on it, but it's not so simple, you need to invest a lot of money in development.
It's definitely not an interesting idea.
