AMC and Regal are probably looking for ways to exploit the customer for more money in order to make up for the money they’ve lost, and will continue to lose without having a summer full of movies for people to go to.
But in all seriousness, they probably don’t want to pay movie studios to use their films for 2 months. Especially older films that wouldn’t get the amount of crowds drawn in, to go along with reduced capacity in itself. The idea of seeing the first Avengers in theaters is cool, but am I going to pay 10-15+ dollars to go see it? Probably not. The whole reason they’d put older movies back in theaters is to get people used to seeing movies in the theater again, and to tweak any safety measures that they can before Tenant and Mulan come out.
Personally, I think if there’s new movies playing in the theater, there will be people that don’t feel 100% comfortable with going to a movie theater that will wind up going. There’s a lot of FOMO in the entertainment business. Shows and movies are all about word of mouth. That’s how a show like Tiger King blows up. People that see movies are very invested in not being spoiled. If Tenant, or WW84 come out, you don’t want to be spoiled of what happens. And in this day and age it’s very difficult to avoid spoilers.