Why isn't wilt considered the greatest player ever?

In his head-to-head matchups with Bill Russell, Wilt almost always came out second best. Wilt was a much better player physically, but Russell had a mental edge, and a toughness that Wilt simply couldn't match. Whenever Wilt led a team up against Russell and the Celtics, his team usually lost and Russell at least held his own in the head-to-head matchup. It's difficult to be considered the greatest player of all time when there's a rival from your own era that you simply could not beat.