Dude, you answered your own question in the opening post.
They continue to make gains on the same program and diet because drum roll it isnt necessary to change programs frequently.
Did you think there was any other reason? Other than the obvious?
Seriously though, I used to the biggest sucker for "scientific" diets and training programs. Long story short, it took me years to realize that those whom I used to look down upon (even though they were making better gains than me)because they ate and trained unscienticly, actually knew more than I did.
Its a hard reality to accept, especially after you've commited so much to one belief system.
I dont want to be a dick, but put aside everything you think you knew, start making your own observations and coming to your own conclusions.
You'll be that much better off for it.