Because its the easiest route at the moment yes but you don't think gun smuggling would be a profitable venture? Seriously part of the reason why the opiate epidemic is so bad is China selling off the supplies for it to the cartels in Mexico, this also isn't to offload the responsibility of this mess on Perdue Pharma. They got the ball rolling and are 100% responsible for starting this mess but you have to be blind not to see how an enemy foreign nation is exploiting the issue and only making it worse to further destabilize a geopolitical rival. Same exact thing applies to Russia and their Interference in the election, they didn't make or start the problem, just took advantage of a fire that has been burning for a while and poured more gasoline into it.
Also again you don't really answer the question of how do you get rid of all those guns. There are 120 guns per 100 people in the US. They aren't going to magically disappear the minute you ban them. You can't just do a full ban, hell I would say half this country wouldn't allow it. So restrictions are the only realistic option.