I think the reason why they don't rev their hardware as frequently as they could is because it doesn't pay to. The number of people choosing between a Mac and a Dell who go for the one with the newest processor has to be vanishingly insignificant.
In my view, people have a clearer picture of how much disk storage they want--the usage meters on their phones help drive that home. Other than that, they are confident that the machine they get today will be faster than the one they bought 4 years ago. But I suspect usage patterns have changed (e.g., more time in the browser; games are played on phones; etc.) so that speed bumps aren't the primary reason why people upgrade, anyway.