Because US dealers and manufacturers (with the US government’s permission) sell them to the Mexican government, where they are stolen from police arsenals in brazen, professionally conducted raids.
Around 02:00 on the morning of Monday, September 27, at least 6 men dressed in CIPOL state police uniforms with tactical gear & riding in a white Dodge Ram entered the State Security Complex (CIPOL compound) & raided the arsenal.
The group entered by the main (south) entrance. They first took control of the radio room. In the process, they disarmed 5 guards & handcuffed their hands & feet (one news source said it was 2 police & 3 security guards).
The commando then entered the arsenal after breaking 2 locks to get through the steel doors. They took 43 H&K G36 assault rifles (.223 caliber), 26 9mm pistols, bulletproof vests & grenades.
They left the way they came with no resistance. Official reports admitted there was insufficient security for the complex.
Hm. I wonder how many of those “3 out of 4” Mexican crime guns traced to the US were sold to the Mexican government?