And yet somehow most people in charge of resourcing and budgeting for projects, teams and companies have some idea of who to hire, how much to pay them, etc. How do you think they do that with something that is nearly impossible to measure?

It certainly wouldn't benefit anyone who hires people if those people could estimate their own contribution, or, god forbid, compare it to their compensation. I think there's a term for the difference which now eludes me.