Even if I'm only DMing and not playing, when I page through a new sourcebook, I always look at feats and PrCs and think "Cool, now I can make my character even more powerful."
As the DM, I don't need new feats or spells or prestige classes, because those things all operate at a level of granularity below what the players actually see. Plus, I can just make up abilities on the fly for NPC's and monsters if I want, since I know they'll probably just be killed by the PC's fairly quickly, so game balance isn't a concern.
Basically, when you're DM, its no fun to min max, because you're already omnipotent.