In prisoner's dilemma cooperating gives both parties the most benefit in aggregate, while defecting gives on party additional benefit at the expense of a steep disadvantage for the other party. Also note that if all parties defect they all end up in a rather undesirable state.

For the SVB bank run, the main loser is the bank, who isn't party to this game (of whether to keep deposits or not). Even if everyone defected, the early defectors don't lose anything. And arguably, it can be foreseen that even late defectors don't lose anything either due to SVB marginally in the "too big to fail" category, or at least, the startup ecosystem being "too important to fail".