Gates, slim, Ellison, the Kochs, and the Waltons were all in the top eight from a quick search I did. None of those gained the majority of their wealth through investing.
The rest of the list includes Ingvar Kamprad, Bezos, Zuckerberg and Amancia Ortega, and none of them gained the majority of their wealth through inheritance or investment either. Now, their kids on the other hand... :-)