GM's have had a serious issue with headlights out on one side or the other. Next time your out just count how many have DRL or others out. Could simply be that there are a ton of these vehicles & many don't realize it or don't care to maintain the burned out bulb.
I too have noticed the same thing with the daytime running lights. I have a neighbor who is local ACE hardware store owner that had this issue, and he never replaced the bulb. Finally, after some years the other side burned out. So now at least they match.