I agree Mexico is hugely corrupt, and perhaps the flow of precursers shifted back to Mexico. But the fact they were sent to the US for years, until recently, because it was easier to slip through US customs than even the notoriously corrupt Mexican customs demonstrates the scale of the problem.
Being able to order kilo of bags full of white powdered controlled substances with Amazon like ease, even if only AAS, and know it had a 99.9% chance of arriving safely through US Customs is almost hard to believe, yet we accepted that as normal. ANY bag of white powder could get through under those circumstances. It's not like it was checked. and the authorities said, "oh, that's just a kilo of a schedule III drug, let it pass.",