I think what you're seeing is that some of the former dominant big men scorers (Duncan, Garnett, career 49% and 47.5% shooters) are now not the 27-30 ppg scorers they used to be (and, hence, not among the league leaders.
There are still some good post scorers. Dwight's scoring 25 ppg on 45% shooting. Jermaine's scoring 24.7 ppg on 47.7% shooting. Elton Brand's scoring 22.8 ppg on 47.8% shooting. And Yao's still Yao.
But the big men who were at the top of the league scoring wise (even Jermaine was a 27-28 ppg scorer for a few years) have fallen off. But I don't think it's ridiculously hard for big men.
But big men without a diversified skillset will shoot low %'s in this league. Interior defense is just far too good for a guy with D outside shooting to succeed. Even look at Emeka, who's been an A- inside since the first year in this league, is only a 44% field goal shooter, and has only shot more than 43% twice in 8 seasons. You need more than post skills.
But it's not just scoring....it's the fg%. Even Yao who has shot over 50% for the past 4 years is down. Meanwhile either we have a lot of slashers that are finishing in the lane...which I don't think is the case as that negates the theory of strong defensive play by bigmen. Or we have an unusally high number of accurate jumpshooters. We have 33 year old Peja and 35 year old Ray Allen having career years in FG%. So age is impoacting the big men but jumpshooters aren't losing their legs.
Yes, there are big men who are still scoring a lot of points but they need more shots to accomplish it. And look at your list....where is a computer generated player that is going to step up and take the place of inside scoring.