White collar criminals in our big banks and corporations have turned otherwise legitimate businesses into vehicles to commit numerous crimes. They use the corporation or other business entity as both a sword and a shield. The entity is used to help commit the crime and then used to protect them personally from any criminal or civil liability. In my experience, more often than not, a prosecutor will forego charges against an individual and just charge the business entity, because it's a stronger case.
If CEOs started going to jail for long stints, that would be very helpful in cleaning up things in a hurry. If the only downside to a CEOs behavior is that he may have to leave his job and suffer some temporary embarrassment, that's not much deterrent to him engaging in activity where he can make large sums of money. While fines can have some deterrent effect on a company's behavior, their effect is often muted by the fact that the fines are less than the profit from the activity, the cost of the fines can be passed on to customers, and in the case of the banks, the fines are subsidized by the government itself or the Federal Reserve.