We call STEM hard since most people fail to learn it to a passable level even when enticed with tons of money.

You can try to explain away their failures to learn in many ways, but the fact is that most people fail to learn STEM well enough to get a job doing it and instead they go to other fields considered less hard. You could say "They just didn't try hard enough" or "they just went into it with the wrong mindset" or "they just need to spend more time doing it" etc, but all of those things are signs that the field is hard.