I think a lot of it is laziness, few want to take the time to build their own business. A trade, a passport, and you can make a fortune off of the Saudis or other wealthy Arabs who want to have someone do dirty work.
A fully licensed HVAC tech can draw his own map. They don’t even need to start a business...just get the training. The demand is there.
You might be on call, you might spend a lot of time in basements or tight spaces frigging with things that don’t work right....but you get paid well.