Probably because the massive amount of regulations that have been added in the last century means starting with nothing is a massive undertaking if you aim at providing a full service bank. Startups are popping up, but usually they only do one thing. And a limited set of that one service at that.
Through all this digitization and cost savings the numbers of banks has gone down. From a infrastructure perspective it has never been as cheap and easy to start up a bank, and yet what we see is the opposite. Now why do we not see new competition in banking?