Think because there is a segment/demographic of buyers that have much higher willingness to pay for certain fares (business reimbursement or personal weath) - even at the slightest benefit - so airlines would leave money on the table if they didn't charge as high as possible (which is the unrestricted first class fare).

E.g. If I was a earning 7-8 figures a year I'm not sure I would care about $1000 vs $4000 flights. I'd just want the best thing possible. To the extent I probably still can't afford private.