Is there a more inhumane method for transporting drugs than putting them inside a living human body?
Every system of border control has a catch-rate that can be calculated. E.g. statisticians can keep track of what percentage of drug shipments hidden in produce are intercepted, how many hidden in agricultural products are intercepted, how many floated in to fishing boats, etc. etc.
That means there is also a catch-rate for shipments hidden inside (poor/desperate/extorted) human bodies paid as border-crossers. If the cost/revenue ratio for shipping drugs in this way is cheaper than other methods, there is an economic incentive to pay poor people to swallow drug balloons and pretend to be migrants traveling for work, family, and other ostensibly normal reasons.
A wall is not guaranteed to discourage trafficking, but it could help. Certainly if there is no wall, there is more opportunity to send human bodies filled with drug balloons by land-crossing.
If the demand-side for drugs would 'just say no,' there would be no money to make by sending drugs across borders, and then we could stop worrying about trafficking. Wouldn't that be nice?
In other words, the drug markets within the US have all the power to stop trafficking in their hands simply by giving up their habits and the transactions those habits engender. That also means they have the power to stop the wall from being built and all the waste and environmental harm that comes with it, not to mention inconvenience to legitimate migrants.