In fairness, MCW and Olynyk were 22 year old rookies and Hardaway and Oladipo were 21 while Wiggins, Parker, Gordon, and Exum were all 19. It's not really a fair comparison.
Why not?
Anthony Davis was 19 as a rookie and averaged 13 and 8.
Chris Paul was 20 as a rookie and averaged 16/8/5
Demarcus Cousins was 20 as a rookie and averaged 14/8/3
Greg Monroe was 20 as a rookie, averaged 10/8
Durant was 19 as a rookie, averaged 20/5
Lebron was 19 as a rookie, averaged 20/5/6
Bradley Beal was 19, averaraged 14/4
John Wall was 20, averaged 16/8/6
Kyrie Irving was 19, averaged 18/5/4
Tyreke Evans was 20, averaged 20/5/6
The list goes on and on.
There are plenty of players who are superstars today, who put up good numbers at 19/20 years of age in their rookie years. In the case of those who didn't (e.g. Harden) it's usually because they never got the playing time. That may be a valid excuse for guys like Exum and Gordon, but it's no excuse for guys like Parker and Wiggins.
The only perimeter guys in the list above who put up scoring numbers like Wiggins their rookie year and became big stars are CP3 and Wall, but both of those guys also put up outstanding assist and rebound numbers (for their position) and the thing that has made those guys so good is their all round game - not just their outright scoring.
When you look at the elite scorers, most of the ones who got big minutes in their rookie years scored at a very high rate from the get go.
I';m sure you can pull out exceptions, but in most cases that seems to be the case.