The lack of welders who can breathe vacuum makes space a challenging welding environment.

They are already using robots to weld starships. Doing it in space removes variables like air, humidity, and shielding gasses.

The difficulty is on Earth you have a convenient stable mass to anchor your robot to while in space everything is unmoored and unstable if you start swinging around a robot arm.

exactly. there are no large masses in space let alone ones we can anchor things to. the country of canada tried to build a robot arm for space and they ran into this exact problem. seriously, if they would just consult smart people like us then maybe they could have saved themselves the trouble of launching silly robotic arms into literal orbit.

Oh no I totally didn't think of the most famous robot arm in space, one that moves slowly and carefully and was planned and designed to work with the single largest and heaviest thing we've ever assembled in orbit. My point was you can't just grab a generic industrial arm and sling it around it needs a lot of consideration on how to mount and move it, Canada arm moves quite slowly and carefully.

I assume you would build in a rail that the welding robot would clamp onto.

no, i think "welding is hard in space" is much more insightful. it is much better to make a broad and sweeping statement and then discard a bunch of important possibilities.