Pros hit the top, then vanish??

How come it seems that when(in reference to pro riders), and in this case I will use Parks, it has been about two three years/seasons ago that he was at the top of the podiums(spl?) #1. Like wise with Murray, Shapiro, ect.. Now you see others there. I'm not saying that they aren't as good as them but IMOP, Parks is the "ultimate" rider along with Harris, can do every trick in the book and then some, well now he's down on the list like 15th or so? maybe injuries? Wakeboarding is not a "million dollar contract" kinda sport so why being so young at age aren't these guys still leading? I went to the Portland pro tour about 2 years ago and when Parks got up to ride everbody got out of there seat to watch him. He is a huge inspiration to the sport. I don't know, maybe I'm just rambling on but it just confuses me, like someone in there working profession, we work for years, some reach the top and stay there till retirement, so I don't understand why these guys don't do this?