What I was alluding to is when you have full access to the cellular network, there are other indicators that can be used to task tracking other than just the SIM card. You don't need to know that phone X is person Y, you just need to know that phone X fits the criteria of how a drug mule operates.
And my point is that this is way, way harder than looking out the window and noticing a radio tower perched on a hilltop that wasn't there before. It's far more clandestine to obtain security through obscurity even if it means operating in a network controlled by the authorities. Encrypted traffic looks like any other encrypted traffic. There's little to make a drug dealer stand out from other cell phone traffic, unless they're communicating over cleartext.
That is the point I am trying to get to without giving away specifics. The reason you see homegrown communications networks in Mexico is because it is trivial for the DEA to identify members of smuggling operations by way of cell phone TTPs. DEA obtains both explicit and unauthorized access to cellular networks in foreign countries. At that point it is a pretty trivial pattern matching problem which allows you to identify persons of interest and track them going forward.
They do use large scale "direction finding" to identify radios used by smuggling operations, but that is primarily targeted at boats with higher power transmitters.
What doesn't make sense is deploying infra in the vicinity of major cities. There's plenty of benign cell phone traffic there, and this kind of pattern matching is not viable.
It's not worth discussing the specifics in the open, but the investigative techniques have gotten good enough that even "anonymous" SIM cards don't buy you anything.