the homebuilt recumbent crowd uses MIG a lot. In fact, there is a video on utube of a production recumbent company using MIG for some operations on recumbents. It's a matter of skill and cost of equipment for the homebuilders. You can do light sections with MIG, but it's not what most people would choose to do given the existence of GTAW/TIG. Thick wall makes MIG a lot easier.

Atomiczombie also has super-thick head tubes, probably for the same reason. The 44mm head tubes that Paragon sells are pretty heavy-duty