Probably because they only have one ticket at the lower price. When you try to book two passengers, they all have to have the same pricing, so they have to move both passengers to more expensive tickets. Revenue tickets behave the same way.
And the same is true for all airlines.
Originally Posted by
nbbb
what is stopping me from buying two individual tickets back to back for the lower price, or transferring points to my spouse's account and buying it cheaper on the exact same aircraft?
You would get one for the lower price and one for the higher price.