TL;DR, these days real-world guns are mostly (if not entirely) used without licensing deals. This is currently allowed under US law as video games are considered "artistic expression" immune from licensing requirements.
You might well ask why the same legal theory couldn't be used for other things, such as cars or airplanes. My guess is video game devs want and need the technical cooperation of vehicle manufacturers in order to more accurately model their products. This isn't really needed for guns.