Banks fail. When they do, those who stand to lose scream for a state rescue. If the threatened costs are big enough, they will succeed. This is how, crisis by crisis, we have created a banking sector that is in theory private, but in practice a ward of the state.
The latter in turn attempts to curb the desire of shareholders and management to exploit the safety nets they enjoy. The result is a system that is essential to the functioning of the market economy but does not operate in accordance with its rules. This is a mess.