The problem is, if you deviate too far from the index, your head is on the chopping block. There is no incentive to outperform the index, and every incentive to not meaningfully underperform it. Anyone bought into an index fund expects exact index performance (whether or not the prospectus technically allows for deviation).
So any manager who values his pay check will say "the index may go up, may go down. The investor's paper wealth may increase or decrease. That's not my problem! And in a market like this, I risk underperforming if I don't own this asset. So of course I'm going to buy it!"
So any manager who values his pay check will say "the index may go up, may go down. The investor's paper wealth may increase or decrease. That's not my problem! And in a market like this, I risk underperforming if I don't own this asset. So of course I'm going to buy it!"