Well, it probably assumes that the 98% are eager to throw themselves in the rat race in trying to become that top 2% or at least closer to them; the fact that they won't get there doesn't mean that there isn't a drive to optimize your career to try.

Great question, and there are multiple explanations that can work, although I disagree with the previous poster:

There is more than one hierarchy of competence. For each occupation, hobby and skill there's it's own hierarchy. So if there's ~30 hierarchies, approximately half of competitors are going to be in top 2% of at least one.

Edit: and if we're talking about purely monetary measures, have a read: https://www.wsj.com/articles/SB10001424052748704409004576146...