In a culture obsessed with individual success, helping someone else does not have any obvious upside, but plenty of clear downsides - what if he gets so good that I look worse in comparison? What if he stays the same and I look like a bad mentor? Why would I sacrifice my time for no practical reward? Etc etc.

Yeah I understand that and I was thinking the same things, but it honestly sucks. I have been in a position where I was supposed to be taught the work on the spot but instead they expected me to know everything and do what I have never done before and it is such a bad experience. :/

It costs money. You're paying that person to be doing something other than working. If you're not squeezing maximal productivity out of your workers, then you have failed as a manager and will not be getting that sweet bonus this quarter