When welding, you put an incredible amount of heat into the material. This causes it to move in real time. A welder needs to predict how it will move ant actively account for this so that the final piece winds up in the right condition. The tolerances depend on the application - for furniture who cares if it's out a bit, for a coolant line to a reactor you need it to be damn near perfect.
Then there's heat penetration - adding heat to the material changes its material properties. Go too slow and steel becomes excessively weak, go too fast and you don't form a strong bond. Again, big differences between "probably won't break if you sit on it" and "this will kill a dozen people if it fails"
Finally, there's just plain old margin for error. If you're welding together consumer products, it doesn't matter if you scrap one every now and then. However if you're trying to join together two multi-million dollar custom fabrications, you need someone who's going to walk up to this and get it on the first try every time.
I'd compare it less to being a major league baseball player and more to being a professional musician - you can't determine that someone could never do it, but some people are certainly born with a knack for it and you can't simply churn out more on a whim.