4. It's Too Big
The epic story is just that... it is epic! Civil War tied in to over 40 books, involved dozens of key players, and covered so much narrative ground that doing it any justice in film would require dedicating an entire "phase" to Civil War alone. Several of the subplots of Civil War could warrant their own films (Spidey's turn, Thunderbolts, Wolverine and the Atlanteans, etc...). When Peter Jackson re-imagined The Lord of the Rings trilogy for film, some things obviously had to be omitted. At the end of the day it was still a narrative success. Civil War...well, it's just too damn big for one, or even two movies! People love epic films and love epic trilogies even more. Even if Marvel and the studios agreed to dedicate that much MCU time to Civil War and managed to work out the licensing, there are a few other issues that would gum up the works...