“Only question is, why not also sue the shooter? He is notably absent from the list.”
And the answer is $$$, no money in it. As you already know.
The failure to do that, however, means a likely failure of the lawsuit as a whole. All the other defendants will blame the shooter and it is virtually certain to be an open and shut case.
If one were to design a theater the emergency exits would be alarmed and turn on the theater lights. That would work in the event of fire or other problems.
Then you allow CC in the theater..and someone who uses the exit becomes a visible target.