I suppose I'm glad you are discovering things through other mediums, but I'm having a hard time coming to terms with why your brother feels the way he does.
Bottom line, I can't recall any time when a game exposed me to something I didn't already know or enjoy. I get excited when I learn that some of my favorite musicians become involved with gaming (I knew of William Elliott Whitmore LOOOOOOONG before his involvement with Red Dead Redemption and Jon Coulton significantly prior to Portal) or when games take cues from films I enjoy. Then again, I'm sort of an elitist when it comes to these things. Sure, gaming is a major hobby of mine, but I geek out on a lot of other things, too, and often times it's more than I do with gaming.
Also, I'm a little baffled that you discovered how great zombie cinema can be only after playing L4D. You do realize that particular film genre existed, I dunno, 40 years or so prior to modern zombie gaming, right?
Even still, I don't much care where other people get exposure from. Everyone has to start somewhere. If you find things that you enjoy, good for you. Keep on doing that and you'll have an immense library of things to check out, read up on, listen to, or watch.