Because, honestly, the MCU isn't all that bloody so far. There was Klau losing an arm in Age Of Ultron, admittedly, and Black Widow used to be a trained assassin, but both of those are exceptions or things that haven't actually been shown on screen. The Avengers are a militarised force and the previous Captain America films show the realities of war, but that's mostly a bloodless affair. In that spirit, the Civil War movie could do with dumping some more of the lead-in that preceded the comics. Namely the part where public mistrust of superheroes has been built up by incidents such as the Hulk going on a rampage in Las Vegas and accidentally killing 26 people. Surely not, Mark Ruffalo! You seemed like such a nice guy! Hulk's off the radar in the films as of the end of Age Of Ultron. It'd be out of character and a tonal shift for the MCU as a whole to re-introduce him in Civil War even if it happens off-screen and the characters simply talk about it by him going on a murder spree. In general, the Civil War comic is far more willing to toss a load of dead bodies into plot. Mark Millar wrote it, after all.