Southwest, Delta, and JetBlue are limiting the number of seats they sell on each flight so there’s no need for passengers to occupy middle seats. Sometimes middle seats will be taken, but that usually means families traveling together. American Airlines has been limiting loads about half as much as others – but that ends July 1.
United Airlines, in contrast, has been happy to sell a ticket for any seat on any flight and fill all the middles throughout the COVID crisis. Starting July 1 even more of those middle seats will be filled.