Stand alone movies

The biggest issue i have with most stand alone movies of Marvel or DC is where are the other heroes during these world crisis's? It isnt practical most times. I understand needing the bases of stand alone movies, but also understand that its not very practical without having other heroes show up even in stand alone movies.

I do look pass this issue and do enjoy these movies for what they are, but struggle with the impractical conception no other heroes would show up.