Aerospace welding is a different beast. Vast/tedious documentation requirements, experience with dissimilar metals, welding gold (think about the cost of splatter with that one), extremely tolerance requirements, etc. Some welding jobs are done under a microscope. You don't fix welds, you through everything away and try again. Very experienced welders report a long and steep learning curve. Hiring somebody from a different industry means investing significantly in their skill set, and waiting a decade for their skills to ramp up. There is also a significant tech component, in that a lot is being automated, where the welder is setting up the robot, inspecting results, and so on, and only doing manual welds where the robot cannot achieve what is needed.

Meanwhile bog standard pipe welding often pays better and the jobs are easier to get with far fewer requirements.

Stennis is not getting its personnel from local grad schools.