Personally I think a big reason is that the promotion is dead. There's a palpable sense in every large organization I've worked for is that working hard simply won't get you rewarded and that there's a hard glass ceiling on your ability to rise through merit.
I've found this to be true when I look at my own earnings as well. I have never experienced a significant jump in earnings or responsibility except by switching jobs. Never. I've worked very hard in some jobs, but I have never been rewarded for it except through an ability to cite my present work as a springboard to somewhere else.