Seems all 3 airlines are doing it, I had only seen about Delta yesterday. When buying a solo ticket you don't get access to the lowest fare classes, but if you do it as 2 or more it unlocks lower fare buckets.

"All three of the country's largest carriers (American Airlines, United Airlines, and Delta) are penalizing solo travelers with higher ticket prices than you can book when traveling with a group – sometimes, significantly higher."

https://thriftytraveler.com/news/airlines/airlines-charging-solo-travelers-higher-fares/