It’s because offences constructed around a primary high volume scorer rarely have serious success in the playoffs due to the ease with which teams can gameplan for that playstyle.

Shaq is an outlier for 2 reasons. First, he was flanked by another major scoring threat, preventing the same level of overhelping in the paint you might see if you replaced him with a more offensively average 2 guard. Secondly, Shaq was a literal one of a kind. Nobody in the entire league was stronger than him, or even particularly close really, and on top of that he was also more nimble and more skilled than 80-90% of centers. Because of this, throw whoever you want at him, he’d still power that shit right through you.

As for Jordan, illegal defence played a major role as his iso-heavy attack couldn’t be countered by soft doubles, as they were illegal. Imagine if you allowed someone like LeBron, KD, or Kobe to isolate without any fear of a clogged lane or covered passing lanes. That’s not to say just anyone could’ve done what Jordan did, there’s a reason the 3 guys I mentioned are 3 of the best scorers of all time in their own right. The Bulls’ offence would’ve had to adapt pretty significantly to such a rule change, though I think it’s fairly clear that even in such a scenario they were still easily the team to beat each of those years.