Smuggling people is actually easier than smuggling drugs (well, adults, either as migrant workers or prostitutes; children are kind of borderline depending on the country).

In a lot of places (US Southwest especially), the penalties for drug smuggling are far higher than for escorting groups of people -- if you get caught, you might go free if it's just a group of workers. The humans can transport themselves on foot. The humans can be held hostage afterward and you can try to extort more money from their family; if a $20k kilo of coke is held for ransom, no one is going to pay more than the discount to an alternative kilo for it.