Demand is a lump (whereas labour is not a lump)

If you have ever spent time trying to work out a suitable birthday present for someone who is relatively wealthy it becomes apparent that most people have most of what they want. In a developed and relatively equal society most of the demands of most of the population are accounted for. They have what they want. For someone who seeks to earn money this is a problem because there is little to supply them with that they do not already have. There are certain things that people regularly buy such as food and to earn money a new entrant would need to be cheaper or more efficient than existing suppliers. But overall it is hard to break into the market because demand is a lump and people avoid spending money if they can.

Even rich people are pretty frugal with their money. Of course there are the exceptions of the 'playboy prince' and the female equivalent but overall rich people don't spend a proportionately greater amount of their wealth than the poor.