Well because he isn’t. Tim Duncan is a superior player. Not by a wide margin however.
Duncan was a killer back to the basket player but also had a solid face up game. Also a tremendous defender who understood positionings and concepts better than almost any other player in history of the game. Duncan basketball IQ is in the absolute top percentile of all time NBA players. He was unlike most superstars, very coachable and had no problem defering to teammates when necessary.
Kevin Garnett had a lot of similar qualities. A tremendous teammates who was coachable. He had very good defensive instincts (although not quite as good as Duncan). Garnett might a slightly better passer than Duncan. His offensive game is more face up than back to the basket,(although he had some face up aslo).
With all that said Duncan vs Garnett debate remind me so much of Olajuwon vs Robinson debate. Hakeem is remembered as a better player and rightly so. But it’s not nearly as wide of a margin as people think. Much like Hakeem, it’s Duncan’s killer back to the basket made him a better performer in the playoffs than Garnett. The game slows down in the playoffs and getting buckets in the half court is paramount.