And unless you have a disability of some sort, or something like it, no one is really incapable of doing any job. It all comes down to drive, willingness to learn, dedication, and pride.
I am a Masters Degree holding Project Analyst who is well respected by my peers for my work. I still earn less than half what my high school degree holding husband makes. He is in computers and his skills are highly sought after, mine are not. It is simple as that. People are willing to pay him tons because his knowledge take time to learn and they want to give him incentive to stay. They know he has options. I am not saying that lower paid positions do not work hard. Their skills are either not as in "Demand" or fairly easy to re-train. It is much easier normally to learn how to be a housekeeper or waiter than a computer programmer.
However, I will tip because of etiquette and the simple fact that they are helping me.