My spare time is worth happiness/distraction to me. Not money. I will confess that in that particular regard, I may differ from some, or perhaps even most, others.
As if that wasn't enough you have a lot of expenses that are flat-rate that you must pay regardless of how you spend your time (such as your rent).
All this is further assuming that your goal is to spend everything you make every month. Most probably put away some money to be able to buy a new car, travel etc. (or even just to be able to repair the car when it breaks)
Well, I've established that I'm willing to work that many hours for that rate. That doesn't mean I'm willing to work more hours at the same hourly rate. Most people I know value their spare time enough to expect a higher rate for working more time.