Utterly Mad

a few reasons
One: They are, just nothing on a large scale. Sting operations, busts, and taking down who they can. However the foot soldiers almost always take the fall, giving false confessions, so the major players almost always walk, and even then they get out of jail relatively quickly (Governor pardons often being used to get them out)
Two: They've essentially bought several people in the government. This generally keeps them out of larger scale trouble
Three: They don't want to kill the higher ups. These guys have enough knowledge that if the government can get them to talk the government will suddenly have a huge amount of information about criminal dealings in the larger criminal organizations, as well as give the government knowledge of a large number of weapons dealers.

And a few other things.
Essentially, they've bought government officials, they've got a large amount of resources to help keep them out of trouble. the government is trying to deal with them, but they have trouble actually pinning anything on major players. And the Syndicate has a HUGE number of foot soldiers who would gladly take the fall for the higher ups.