Because very few people actually pay those prices. F (on flights where a 1-2-1 lie flat C is available) are likely sold to a few who have enough funds as to not care, in which case they bought F to begin with.

Most others are only in F if using miles or found some super deal.

I was once in SQ F across the Pacific (miles). It was very nice. But I would not have paid Y6000 one way to upgrade from C to it. And from what I saw, the MU F was not the SQ F.

Admittedly I lowballed it when I tossed out $100. But I could not imagine even thinking about coughing up $1000 or so to move up. I would be curious if they ever get a taker on that deal.