Why can't horizontal stabilizers be longer than they are today? I've wondered this question because if the horizontal stabilizer is longer than that means more lift. My guess to this question would be the result of wing tip vortex strength on a longer wing
The horizontal stabilizers can be longer, they just don't need to be any longer than they are.

Each additional square inch will add induced drag and parasitic (form / profile) drag which costs fuel so they're not made any bigger than necessary to provide adequate control of the plane.