I think the point is that they don't have the influence to intentionally deviate from the index because they think they know better. If your mandate is to be passive, then you need an index to follow. If you are that sure the S&P 500 index is wrong for some reason, or whatever other index you follow, then you need to invent a new index. Then, you can follow the new index.
At least my index funds do that. They don't get to constantly trade like non-index funds do, and they typically stick with the index, but every index fund I own has a line about "we select stocks that we think will match the index", which is different from buying the stocks from the index.
Again, the vast majority of the time they are matching the index stocks. However they have the right.
I guess that's true but put yourself in the position of a major fund manager. Would you rather explain "We lost 3% this year because of a dumb IPO because we track an index that includes dumb IPOs," or would you rather say, "We lost 3% this year because I decided, as a passive fund manager, that the index was wrong and I knew better"?
Your career would be over.
Or at least, you would have to transition over to an active fund!
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