Earlier quoted context omitted.
This is for the same reason they don't mix gold ETFs into their indexes. The point of their funds is to track companies that produce goods and services, not asserts and commodities.
Twist things around however you want, but VGPMX [0] invests in the companies producing precious metals. 1/4 of the portfolio is that. [0] https://investor.vanguard.com/investment-products/mutual-fun...
For instance, Vanguard is one of the largest institutional investors in MSTR and MARA.