The only reason I can think that they're doing this is the World Cup and they want to keep their noses as clean as possible. My guess it will disappear just before or just after the WC.
It's completely bonkers though. I bet there are a lot of people who would willingly extend their layover to avail of a hotel or other revenue generating activities in Qatar if they could - but won't if it involves either pre-testing or a antigen/lateral flow test within 24 hours. I certainly could with my recovered status but I can't be arsed with that arrival test.