I think it's usually practicality, a book can include things like that because the author can just toss it in as soon as they think of it, and devote a paragraph or two without it seeming like an info dump or similar. Exposition pieces are always a gamble and unlike a book the audience doesn't pause to absorb and recall something then continue reading, the film keeps rolling so you need to keep the reminder up long enough for them to recall. "Where's the calvary?" is also usually a bad question to raise most times because as soon as you remind them about War Machine they're much more likely to wonder where Captain America or Thor are, or start losing their acceptable break with reality and wondering why there aren't hundreds or thousands of troops in these things, since he's a reminder they aren't unique.
Not that they couldn't do it, and well, but if you think about other cases what tends to happen is Fury and/or stark would be standing in front of a big screen showing recaps of video of the hero or a series of picture profiles of him and other possible calvary. That tends to be a hamfisted, like most exposition, where the people involved are discussing something they'd obviously both know already or some aide is, in the middle of a crisis, taking the time to do Powerpoint slides for someone who knows everything on there. Not they it can't be done well, Stark could be getting his but kicked and they could slide in a moment of humor by having him call Rhodey for help in-suit and show video of Rhodey sitting on a beach with Umbrella drink, which would need a cameo, or patching through to an office phone ringing without him there, straight to answering machine, pictures of Rhodey with family and friends and in War Machine get up sitting around, but the answering machine bits don't work so well in cell phone era. I'm sure they could think something up though, I don't want to imply this is some sort of massive hurdle, just a big enough one that a screenwriter is likely if they thing of it to bypass it as an extra hurdle if they can't think of a good way to do it.
- Albert Einstein
King of Cairhien 20-7-2
Chancellor of the Landsraad, Archduke of Is'Mod