Because Duncan was 37 at the time and while still very good, I wouldn't consider him dominant during that postseason. It's not like he carried that Spurs team; it was so balanced and had phenomenal teamwork.
He might but his effectiveness would be limited in the playoffs when teams/defense got real and his team wouldn't win. This is Shaq we're talking about, whose team got swept out of the postseason six times during his career even with old school post-friendly rules. They'd throw a hybrid zone up all game, overload strongside pre-catch, and make entry passes impossible. If you want to stop someone from posting up today you can do it. You couldn't a dozen years ago (well, you could but defenses hadn't fully figured it out). Not to mention there'd be so much flopping. Or they'd go the complete opposite like teams did with Embiid last postseason and let him get his on the block while sticking to shooters since it meant Philly would be taking fewer threes - it was a trap and Philly fell for it. Shaq's team is scoring twos all game (while he bricks plenty of FT) while you're bombing threes at a standard modern NBA clip. Good luck winning that game.
I wouldn't call the Spurs or Mavs absolutely loaded. Nor Cleveland, really. It's more about how offense is run: perimeter-centric vs an inside-out big man offense.