I don't know. I would guess that some factories do it that way.
- Most certified welders are familiar with structural welding in thick metal; fabrication of thin metals like sheet metal, but not in heavily loaded situations; and mild steel not chromoly. Of course there are welders familiar with structural thin tube welding in light guage steels, like aircraft welders, but it is much less comon.
-the problem is that arc welding creates high temperatures that reduce the strength of the underlying tubing, normally this is dealt with by limiting the welding to the ends of tubes where they are thicker. Possibly a manufacturer of cheaper frames has a prcess where the bosses are machine arc welded to a fork, but those may not be high performance tubes. If your bike has high performance tubes in it they probably aren't up for arc welding.