I know that's true, but I'm wondering - why? Why wouldn't they just withhold their vote and let the remaining (active) investors make decisions?

I don't know where you get that idea from. They own so many shares they have direct control over who gets appointed to the board, and unlike a small investor when these guys walk away with their money it hurts. I often see the news reports of them flexing their muscles in the board room.

It's true they probably don't have much to say about bets like 18A or corporate culture. But they will almost certainly be involved on the decision on if or when Intel will be split up - if only because these investors decide which, if any of the new entities they are prepared to fund.