He is though. Undrafted. Thought not to have much of a chance in the league and yet still fought his way into the NBA (on a serious playoff team no less). Over 35% of NBA players have played in the G-League at some point or another. The Spurs for example, they expect their rookies to play at least primarily their first season in the G-League. The only one who hasn't really done so is Kawhi Leonard and that's fairly obvious why.
Kris Middleton, Pascal Siakam, Rudy Gobert, Clint Capela, Robert Covington, Quinn Cook, Alfonso McKinnie, Thomas Bryant, Yogi Ferrell, Jonathon Simmons, Tyler Johnson, Justin Holiday
All of those guys got better in the D/G-League. Three of them were first rounders (Pascal, Rudy and Clint) and they still had to do long stints down there. Now they are all very rich men (Siakam is about to get the bag). If you go to the G-League, you have to ball out. It is a prerequisite.