I don't really think this assertion is true for all kinds of works. While there is an element of truth in this, I am equally sure of the fact that there are some physical works that pay higher than mental works. It actually depends on how we choose to look at it. A lot of people are ready to commit themselves to physical works while they are okay with whatever they are given as salary at the end of the month based on the agreement.
However, just a few people are ready to engage in mental works just because it can be tasking at times. Mental works seem to be paying than physical works because few people are into it, and that's why there are so many opportunities for those who are good at it. For example, there are thousands of people who are ready to compete for just 500 opportunities for physical works, whereas mental works have not gotten this level of saturation.