The normal explanation is that the happiness derived per dollar decreases with every dollar.
The idea being that a couple making $200k jointly per year spending $800/mo on a maid gives them far, far less happiness than a homeless person getting their first warm meal and shower this month.
A working couple with young kids getting a regular maid for the first time can be life changing. It's not the same as your homeless example, but may be easy to underestimate or trivialize.
