It's mainly because the FCC is a government agency. Which means it's filled with lots and lots of bureaucracy.
It's not uncommon to take years to get a pirate shut down. Why? Because so many steps have to be gone through, and the agents are overworked and stretched way too thin. So a lot of time happens before being first sent notice of liability and marshalls kicking down your door. I can recall two instances off the top of my head where a pirate was on the air for a year, and on and off for over two years in separate areas. Both cases both were served notices, fined, and threatened but not much was done.
This has prompted legislation in Florida, where there was a major problem with pirates in cities like Miami, to allow pirate broadcasting to be dealth with on a state level. Therefore, the local sherriff's office can kick down your door in a short amount of time. I am not sure what the latest is on this law, but I know there was a lot of back and forth on this because it's state officials handling a federal matter.