The simple fact that Harden puts other teams in instant contention, but not the Spurs. I don't see NO doing that trade, but Boston surely - a team spearheaded by Tatum and Harden is an instant contender, perhaps even le favorite. The same goes for Philly with a pairing of Harden and Embiid, that PnR plus the rest of Philly's roleplayers can definitely reach the conference finals at the least. The Spurs with Harden, on the other hand, are a middle-of-the-pack playoff team waiting to get bounced. We're too early in our rebuild for those kinds of moves, not to mention they'd gut what little potential-filled players the Spurs still have - and I have little interest in making the playoffs one or two seasons with no real hope of advancing, only to have Harden leave in FA. Hard pass.
I mean, Jaylen Brown could pretty possibly be on the move as well. Simmons isn't the only package, though certainly the most likely one, especially given the Morey connection. And I assure you that the Rockets consider Ben Simmons a "true centerpiece", he's been on every report of Rockets demands as the picture-perfect example of a return they'd want (true, not what they're necessarily going to get, but that's irrelevant to this point). Simmons > any package the Spurs (and other teams) could offer.