In a society, different occupations carry different levels of prestige and different economic rewards. This system of inequality is called social stratification. Sociologists belonging to the functionalist school argue that certain jobs require more skills and specialized training and are more difficult to learn than others. Such jobs are rewarded as they are difficult, require long periods of training etc. So rewards have to be given to encourage them.
The functionalist school to which Davis and Moore belong, argues that economic stratification exists because it meets societies' needs for productivity by motivating people. Some jobs are more important than others. They need greater responsibility, greater skills, longer work hours etc. So they are rewarded better.
Conflict theorists like Tumin argue that inequality exists because the wealthy and powerful make the social system work in such a way that it protects their interests. They say, some of the better paying jobs are not necessarily crucial to the society. The society needs a farmer and a garbage collector much more than a lawyer.
Wealth, which is inherited is distributed unequally than income. It is easier for a person born in a wealthy family to train as a doctor than a person from a low income family. So it is not motivation alone that drives people to choose seemingly difficult professions which enjoy high pay and prestige. The stratification system of most societies endure a pattern for a long time.