The sks's and tapco stuff you see confiscated near the border is clearly from the US. These arms are from "bottom feeders" - gang members that acquire guns through theft or straw purchases. The "bottom feeder" gangs are what the cartels use for distribution at the local level as well as cross border courier work. These groups are considered expendable and replaceable by the cartels (cannon fodder) and as such do not get the top notch weapons, gear or comms.
Further up the Cartel food chain is where you'll find the nice hardware, occasionally these military weapons, vehicles, gear and comms are apprehended but never in any quantity. At the higher level in the cartels, there is much cross over from the law enforcent, government and military and hence the serious firepower is only used or needed when a turf war erupts or a power vacuum exists.
In Mexico today; the various cartels, military, law enforcement, political structure are one in the same. A container full of coke coming north can easily have FA guns and RPGs included in the shipment as well as acquiring US gov supplied arms from the Mex military... there is simply no reason to use that kind of firepower to invite the US to completely seal the border, that would be bad for smuggling/business. Stolen Tapco works just fine in the hands of gang bangers.