One small note: welding metal distorts the metal. Not my area of expertise, but I know it needs to be dealt with and dealing with it adds cost. Pole gets around this for some of their frames by milling them from a billet, but that is not a low cost item. As others have pointed out, the issue is the price point set for the frames.
As far as QC goes, an automated vision system can inspect each frame in a few seconds. I have been involved in a couple such systems for other products ranging in size from a gasket under a 12 mm diameter Belleville washer to an airliner wing. The costs are in developing the system and the shop floor time, The Non-repeating Engineering Cost needs to be amortized over the production of the frames and the shop floor cost is a direct manufacturing cost. As others have said, not difficult. It just adds cost.