It's pretty obvious why when you watch the tape. He literally just stands there if he's not initiating. His screens are a joke.
Now, one might say "oh, well he obviously needs to rest if he's initiating on the other possessions." To which the obvious response is "huh, maybe my-turn-your-turn offense isn't actually as valuable as holistic offense, including screening and movement, that stresses the defense more continuously?"
I will die on the hill of Tatum being a better offensive player than Luka, particularly if you have good surrounding players. Personality....quirks...aside, I think Tatum+Kyrie would destroy the league.
Durant has a similar issue to Luka, in terms of his overall impact being less than you'd think based on his numbers, due to not stressing the defense as much when he's not initiating.
Watching Luka makes me wonder about LeBron. It's easy to watch Luka and say you can't win with that much heliocentric offense. But it's interesting to think about where the tipping point for an offense is along the heliocentric continuum of James Harden to LeBron. I think Luka falls somewhere along it.
Obviously, prime LeBron being a defensive wrecking ball has a lot to do with the playoff success of those teams versus the Harden Rocket and the Luka Mavericks, but there's a big difference offensively as well. Every LeBron team, regardless of the coach or the players around him, has essentially defaulted to LeBron holding the ball and making a play. Depending on the cast around him, there might be more or fewer one-on-one opportunities for other guys like Wade or Kyrie, but everything has always revolved around LeBron, and it still does, even in his age-39 season. Historically, those have been very good offenses, until the last few years. Part of it is I think what you point out about Luka - he doesn't move without the ball, he doesn't screen, etc. LeBron has settled into that as well, and I think the offense as a whole has suffered as a result.
Point being, you can be a functional offense, in both the regular season and the playoffs, by running everything through a guy who is an offensive ecosystem unto himself. But that guy has to do more than just score and set other guys up. He has to screen, has to move without the ball, etc. I'm not sure we're ever going to see Luka do more of that than he's already doing, and it's possible that team can't win a championship this way, regardless of who they surround him with.