Some might just be personality? I think many hackers have a cultural disconnect with the traditional aspirations of wealth, because many of their/our aspirations are hacking-related rather than wealth-related. What would I do if I could do whatever I want? I'm not personally very excited by the possibility of staying in 5-star hotels, drinking expensive wine, owning a yacht, having a big mansion, etc., so those aren't really goals. I am excited by the possibility of being able to work for extended periods of time on my own projects without worrying about their profitability though. So that's getting-rich aspiration in a sense, in that I want enough money in the bank to bankroll 20+ years of carefree hackery, which is actually quite a bit of money.

I'm not sure if it counts for your question, though. In a sense you could say that I don't aim to "be rich" in a qualitative sense, but to "be middle class for a long time without working". And you may be asking why more people don't aspire to be rich in that qualitative sense? On the other hand, a person who can live a middle-class lifestyle for an extended time without working is objectively pretty rich...