To become financially independent.
Several people have looked into how people become wealthy, and they divide the methods into 5 broad categories.
The categories(*) change depending on how you slice them, but generally the two which are accessible to everyone are: 1) Commission sales, and 2) Starting a business.
Commission sales is for things like IBM mainframes, telecom equipment or movie scripts, where a single sale can net you 6-digit commissions. A number of people have become independent doing this.
If you can run a successful business you get to build up equity using tax breaks and the productivity of your employees. It's not unusual for someone to start a business and sell it 10 years later for several million dollars. (BTW: The most common business that makes one a millionaire (as of the several years ago, may have changed) is dry cleaning.)
Being financially independent requires roughly $1 million in assets. If you put that in a long-term equity fund, account 1% for management fees and maybe 2.5% for inflation, then you can retire and pull out $50,000 a year for the rest of your life.
Different people have different needs (family, kids, lifestyle), and different levels of economic risk (I'll need $2 million, just to be safe), but that's the basic formula: Figure out how much you need for a comfortable lifestyle, figure out how much you need in the bank to supply that lifestyle, start a business and build up equity until you get that much, then retire.
More info: 80% of first businesses fail, but only 40% of *second* businesses fail, and the percentage goes down fast after that. Having business experience is apparently a strong predictor of future success.
(*) Other categories are: Inheriting it (3/4 of the buillionaires), marrying it, *other* (really rare things like winning the lottery or finding an unknown Botticelli in the attic), and so on.