It only looks that way if you ignore that they're playing more minutes in the playoffs. Their per-36 scoring numbers and their FG% go down or stay the same:
MJ per-36 in regular season for Chicago: 29.4ppg on 50.5% shooting
MJ per-36 in playoffs for Chicago: 28.8ppg on 48.7% shooting
Shaq per-36 in regular season: 24.6ppg on 58.2% shooting
Shaq per-36 in playoffs: 23.3ppg on 56.3% shooting
Duncan per-36 in regular season: 20.1ppg on 50.1% shooting
Duncan per-36 in playoffs: 20.6ppg on 50.1% shooting
Dirk per-36 in regular season: 22.2ppg on 47.2% shooting
Dirk per-36 in playoffs: 22.4ppg on 46.2% shooting
Wade per-36 in regular season: 23.3ppg on 48.1% shooting
Wade per-36 in playoffs: 21.3ppg on 47.4% shooting
Lebron per-36 in regular season: 25.3ppg on 50.3% shooting
Lebron per-36 in playoffs: 24.8ppg on 49.1% shooting
Durant per-36 in regular season: 26.4ppg on 49.1% shooting
Durant per-36 in playoffs: 25.5ppg on 47.1% shooting
Kyrie per-36 in regular season: 23.6ppg on 46.5% shooting
Kyrie per-36 in playoffs: 23.7ppg on 46.5% shooting
All of those guys saw their per-36 scoring numbers drop in the playoffs except for Duncan, Dirk, and Kyrie, and those three only increased by a fraction of a point. And ALL of those players saw their fg% drop or stay the same in the playoffs.
I only found one star whose numbers actually went up in the playoffs:
Hakeem per-36 in regular season for Houston: 22.3ppg on 51.3% shooting
Hakeem per-36 in the playoffs for Houstong: 23.5ppg on 52.8% shooting
That's a pretty impressive jump.