You just said it yourself, "tunnel" & "border crossings"! You put a kilo of coke in the illegal's backpack, they walk over the border. If they get nabbed (unlikely) it's their ass, not the cartel's. That's part of their payment to the coyote & the cartel, running drugs.
If you can get a person over the border you can get drugs over with them.
You've never seen a news story, documentary, etc., about people swallowing balloons of heroin?
C'mon man, it happens all the time.