The steel that an experienced certified welder works with isn't bicycle tubing. A master pipe fitter, for example works with pipes that have walls around 12mm thick. Bicycle tubing has wall thicknesses of tenths of a millimeter. The thin metal doesn't weld the same and is very easy to burn through.

And a "experienced certified welded" may not work at "most autobody repair shops".

That's where I found out about how difficult bicycle tubes are to weld. I had a machinist who welds pressure vessels, fabricates delicate equipment for scientific experiments, is a bicyclist and whose welding skills go far beyond being simply 'certified'. He was astounded by how thin the steel tubes of a bicycle are and he commented that they would be incredibly easy to burn through. That just about anyone with a welder can fix a steel frame is a myth. Any idiot with a welder can burn a hole in the tubing but that's not fixing it.