Because it ignores free throws, which use possessions just like shots do. TS% already factors that in free throws, and when you look at that, you don't get the misleading numbers.
You're misinterpreting the stat. It's not the players individual PPP multiplied by 100. It's the average possession for the team when the player is on the floor multiplied by 100. Of course Kawhi is on par with Harden and Prime Melo. He spent half the season dominating the ball and is still the go-to guy.Per 100 possessions, Kawhi's scoring efficiency is on par w/ what Harden did last season when he led the league in TOTAL points & he's getting to the line at the same rate as Carmelo was at the same age.