i'll give it a shot. i own a business that could potentially make me rich if i sold it, but haven't considered it yet. (it could also make me go broke, but that's another story).
a poor or lower middle class person would probably disagree if they saw my income and business bank accounts, but hey, everything is relative.
anyway. here are some reasons that may or may not apply to myself and my group of peers (all business owners end up hanging out with a bunch of other business owners or sales-type people, it just happens).
1. they like working. they may deny it, but they do. they like making things happen, making things in general, etc.
2. they feel like if they stop working, the money will disappear, or be taken somehow. this may or may not be rational.
3. they are compelled to do it, it is simply an inner drive they can't overcome. i.e. that's how the money was made to begin with.
4. they like having a high level of disposable income to spend on nice things/experiences/influence. this is the polar opposite of how a lot of people on HN think, who seem to constantly trying to humble-brag about their rankings in the frugality olympics.
i definitely fall into #4, the others maybe, maybe not. i like nice shit, so i work hard. if i wanted to not spend money, i'd just be poor and not work much. i did it for 4 years in college, i could do it for the rest of my life if i wanted to. it's not hard to do nothing and spend nothing. it's literally the easiest thing in the world, you just don't do anything, all day long.
sorry for being snarky.