Yeah, but we also have to dedicate a lot of resources to MAINTAINING our bodies. Seeing as Zombies don't do that either, you're still going to see a lot of rapid destruction. Just walking a long distance, especially under the condition of being already dead, creates a lot of tiny tears. When you're alive, you can repair them pretty quick, and just take time off to relax and heal. Zombies do neither of those things, constantly damaging themselves without repairs. They can take the damage, but pretty quickly they'll start to having difficulty moving, muscles having been damaged so badly they just can't move. Not mention them starting to dry out.
And, even if we go with your marsh gas theory, that's still a finite amount of energy. And would be a very poor survival trait, what with the exploding and all.
Which kind of reminds me of one of the reasons I hated Frankenstein.
t:
Let's skip all the re-animation bull-shit for a second, and all the man-monster things too. The Creature taught itself to read, talk, and be sophisticated, through observation alone, in, what, a year? Fucking BULL. SHIT. No one learns that fast. No one. Furthermore, if Frankenstein wasn't such a freakin' idiot, a good chunk of the latter part of the book could have been avoided. Frankenstein could've revealed his research to the world, and become the fucking new age god and posterboy for science. Or, admittedly, been strung up and killed for crossing several lines, like sewing together corpses, robbing graves, and stepping into God's territory. But you know what? That would have been interesting too.