Beyond a small minimum requirement, turning the crank more only leads to the expectation that you will continue to turn that crank that much. Rewards for going beyond -- money, security, autonomy -- are rarely present and almost never in proportion to how much you turn the crank. Plus, one day the company will decide it no longer needs you to turn the crank anymore, and without so much as a "thank you" you're on your own.
People only have a finite amount of 'caring' to give out. Why invest a lot into something when you feel you won't see any difference for your effort?