Imagine Anthropic had gone public a year ago when it had a $61.5 billion valuation. Index fund investors would be up in arms demanding this change after missing out on the 15x run to $965 billion.
The point is that that’s a valuation on paper, made up by a small group of investors that have a strong incentive to have the number be high. Would the number have gone up like this if it were a public company?
Who knows, it could have gone up more. Sandisk just did 40x in a year as a public company. That's the point of an index, buy everything so you don't miss out on the outliers that provide most of the return.