I agree, for many years have wondered the same question. Apparently plenty of drugs come across at the actual border crossings, the volume of traffic and clever hiding places overwhelm security.
Chasing small time dealers will not end the opioid traffic, start at the manufactures. Where do meth components come from, find that and cut them off. I know sounds so simple, but look at the failed "Drug wars". Anyone whom wanted weed has been able to get it since the 1960's, after 15 years in Afghanistan they are growing more opium than ever, alcohol did not disappear during Prohibition