Simple except that some banks are so big and entangled into the system that if they go down, it would start a chain reaction that would take down the entire financial system, destroying other banks, creating runs, destroying pensions, toppling industries and ultimately collapsing the entire world economy.
This isn't an ordinary situation with two lemonade stores competing against each other. These are companies that have gotten so huge that entire governments depend on their existence. The consequences of their failure is so significant that countries are forced to essentially pay them if they ever get in danger of bankrupcy. Hence the bailouts.
The "fundamental rules of capitalism" don't allow a company to profit exorbitantly to the point that any failure can tank the entire system. Reward requires risk. And the heads of these companies know that there is no risk really-- not to them personally because they have golden parachutes and bonuses that pay out before the cumulative damage to their companies is done-- and not to the company, because existentially losses will be covered by the government/people to avoid the destruction of the world economy.
So it's not so simple, really.