From what I've read the Cartel members operating in the U.S. have learned from the mistakes the Colombians made in the 80s to buffer themselves from people in thier organizations and networks who are not trustworthy (or simply scared shitless of them) people they've known from Mexico. If someone from Mexico operating here crosses them then they know their whole family back home can be easily wiped-out, doing that to a U.S. Citizen is a lot more messy and draws LE heat.

I'm sure that the DEA and maybe the FBI have undercover agents and plenty of CIs in the street gang distribution networks, but that's steps down the ladder from cartel people operating here. At best they may be one or two people down the chain from actual cartel people here. Mexico itself is a no-go zone for LE Agents (unlike Colombia or other parts of South America) because they simply cannot risk trusting the Mexican authorities to work with them, law enforcement and local government down there is litteraly an arm of one cartel or another.

The cartel's are smart and they know if they start any violence the like of which is going on in Mexico with decapitiations and gunfights then the Federal U.S. LE will come down on them like a ton of bricks here and put even more pressure from the Mexican government even more than they have to break them.

They stay in the background and let street gangs take the heat on the streets since they've made their profit once the product has left the first level of distribution once it's in the U.S. It's hard to get to them.