Maybe a simple answer but on flying back from Singapore this week the WT+ cabin I was in had around 9 empty seats. Why do BA not actively sell these pre-checkin or even at checkin?

When they appear to be cutting cost at every other turn this seems a simple way to increase revenue.

I heard one of the crew saying they like to leave a seat empty incase they have to move people, however leaving that many empty seemed strange to me.