Difference in skills is clearest in games. When you can see someone winning and someone losing you can know who's better, despite perhaps not understanding why the winner won.
It's only totally clear in one on one, perfect information games like chess. Take a 5 on 5 game with hidden information such as League of Legends and people come up with excuses for losing: bad luck, bad teammates, bad game balance. All it takes is a little bit of ambiguity for people to seize upon an escape hatch for their threatened ego.
I think a variation on this is a key factor of life and culture in engineering - at some point, either what you made works, or it doesn't. You can argue, debate, philosophize and talk, but at the end of the day - there's either a working thing someone else can look at, or there's not.
(I imagine this is true in other places - sales, business, design - it just gets increasingly hard to tell "if it worked")
You're right, but sales, business and design have too many variables involved in the apparent success of what you make, so you're not able to safely verify what "worked".
Even in engineering, two people can make something that "works", but which one is better? Maybe the worst product had the better marketing, so it sold more and in the end there are more people praising it, then you'll think it is better.
It's a filter, not a metric: you can say this product worked, as in it did what it said on the tin, but the worse/better comparison doesn't work out.
The other aspect of "at some point you can just build it and it works or doesn't" is that when it's built, you can (ideally) just go read the code and find out how it works. It's ye olde issue of "it would take me almost as long to explain it as to make it..." - if I'm having trouble communicating an idea, I can just go make it, and then show it to you.
