You sort of right about staying the same but Jordan Hakeem, Shaq, Duncan, Dirk, Kyrie, Durant, Wade numbers all got better in playoffs. Plenty more players but you get the point.
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.