They are not irreplaceable but it would create the sort of inverse "moral hazard" that might stop these practices, which are apparently what's being protected.
The revolving door between the financial sector and our government- if they jailed people now then when it's their turn to get rich off of their government connections they might risk jail.
It’s bad for campaign donations and bad for future highly paid job prospects in the financial industry. So politicians and agency workers have strong incentives to not be too tough on banks.