Consider this; a Black Market is, in effect, the same as any other market - except that it isn't regulated or watched over by any legitimate authority. There is more money in the US, than in e.g. Mexico. Pure and simple; and this is incentive enough for a lot of people to go there. Once they get there, they will find some means of getting a "piece of the pie", one way or the other. And since they are cut off from all legitimate markets, they will be forced to seek relevant services elsewhere. If the immigrants are willing to pay, don't you think that someone will be willing to provide?
Drugs are illegal; yet they somehow miraculously appear in the US, and many US citizens happily use them. How come the prohibition hasn't worked?