Having worked in both low end and high end job I have two answers to that. The first is social status. No matter how much you earn, you will always be seen as more intelligent and more "worthy" because you have a higher status job. I think for a lot of people it's this admiration that would be enough. The second answer is physical reasons. My sister's back has been shit since she was 30 because of her endless standing in a barista job. Some jobs are insanely hard and just the comfort of being able to take a coffee break and a chat and sit at your desk is absolutely worth striving for. (There are people who enjoy manual and physical labor, but then again, why punish them financially, if they are willing to do the "harder" jobs?)

And yes, a third answer would be the urge to learn more and be more.engaged mentally. But wanting to do that and that having to do that to achieve success are two very different prerequisites.