Well, to be accurate, these vessels are usually not fully submersible / true submarines. That would require a level of engineering and cost that is probably not worth it for smugglers.

These are semi-submersed "snorkeling" vessels that breath surface air and ride just under the water, which makes it much simpler to design/build + operate.

The real goal is to get most of the ship off the surface where it can be easily detected. Semi-submersible is good enough for that purpose.

I'm guessing various countries' navies (underwater sound detection) probably can detect these vessels with some accuracy if they wanted to, they just don't have the resources/time to go after them as their top priority.