The post up is inefficient because teams are allowed to double more freely, teams are much smarter about doubling, and the rules allow for physical defense on the post but not on the perimeter. Offense overall is historically higher than its been, because of the rules and how teams are exploiting them.
Add in the increasing skill in shooting 3s across the league and the post is no longer as efficient as other actions. Typically the highest value outcomes of a post up are:
1. A dunk
2. A kickout that results in a wide open 3.
When the Celtics post up, they often are trying to draw help so they can get the pass out and get a good shot via that. The team's most desired outcome is the pass!
Anyways most of the 'best" post scorers by PPP are low volume guys. The players are rather good at it of course, but they also are only posting when they've got really good position or a good matchup.
Embiid is the massive outlier who is the exception that proves the rule.
This is all true but to add onto this, you also need to understand that the art and skill of low post big men has been decreasing for decades. Big men were drilled from the time they were in 9th grade through high school, years of college and the pros the art of boxing out, anticipating the direction of rebounds, establishing initial position, proper footwork, proper balance, proper dribbling technique on a back down, hook shots, down and under moves, using the rim to fend off a blocked shot, proper use of the backboard, kickout to an open shooter, passing out of the double team, etc., etc.
As the game has evolved, those big kids aren't having that game be drilled into them any longer from a young age. Those kids aren't practicing and learning those skills. They also aren't using them at a developmental level as much either because the best bigs are in the NBA after no time or one year in college as compared to 4 years previously. And when they get to the pros, they are taught the outside game as well as the down low game because those players are expected to play the whole half court, not just in the paint, giving them less time to learn down low stuff.
What this means is the skill that would make post players more efficient are not stressed or, depending on the move or skill, even taught any longer. This is definitely a contributing factor to the post shot being less efficient as a whole as the players aren't having those skills made a priority anymore so the average low post player isn't as good at it.