> Is this not exactly why banks have to deposit X amount outside their doors?

Sorry, what do you mean?

Basically, the historical origins of different bank regulations are quite complicated and also differ from place to place, and they were often put in place by (political) coalitions of people who already had not one single motivation but a whole spectrum of them.

And then in addition to historical reasons, you also need to ask why things are still in place. Inertia is one answer, but it's not the only one.

Where did you get this idea?