Well actually, it's more because his shot volume is pretty much bang average for a wide forward. Those two were just examples from players who play in similar teams. Cunha isn't actually all that better at shooting than average, certainly this season his conversion rate is outright deceptive. He's disgustingly over-performed this season to a level nobody is going to sustain (he's the league leader in this with almost twice his xG), he's pretty average at finishing if you look across a wider sample size. Though it's worth highlighting when he was Garnacho's age he was similarly under-performing, it's not actually all that uncommon. He finished below his xG in 5 of his first 6 seasons.
Except that's not the assumption, nobody said he will automatically do anything. You've actually touched on the solution yourself (partly) below in the second bolded bit. While I fundamentally disagree that a 20 year old winger isn't going to improve physically, I think it's pretty much guaranteed, there's a lot of things he can do (like making more intelligent runs with the ball across defenders rather than trying to run straight past them) or, as you said, actually start running with his head up more so he can attack space rather than being shepherded into dead ends, that will help him significantly in all aspects of his game.