There aren't many billionaires, full stop. (A little under 2000.) And if someone (say) inherited $500M or earned it by building and selling a very successful business, and then put the money into index funds and waited for it to grow to $1B ... I don't think we'd say they became billionaires through index funds, we'd say they became billionaires by inheritance or by growing a business.

Trump became a billionaire by inheriting a fortune. The fortune he inherited was less than $1B, and it happens that the path he took from <$1B to >$1B involved running a business, but he could have invested his money conventionally and done just as well.

I would expect that few billionaires have their wealth invested conventionally, though. To get really rich you generally either need to do something exceptionally lucrative or inherit from someone who did. In the first case, (1) the chances are that you have a drive to keep doing exceptionally lucrative things and (2) you're likely to think -- with some justification -- that having done so well for yourself you can do better by carrying on than by just investing and relying on other people's success.

In the second case, the chances are that a lot of that inheritance is in the business that made your family rich. Again, if it's been so successful you're likely to think it better to keep most of your wealth in that.