They generally do so because it's over a shorter time span, though. When you just need to show the infection and then a week or a month of running panic, the long-term issues don't come up. And you usually just either kill off the main characters, or the surviving 2-3 head off into the distance in a car, or a boat, or whatever. It's somewhat rare that you're dealing with a situation several months, or in the case of TWD, about a year after the initial event.
You're right, though, usually a lot of hand-waving about the How and Why of it. Resident Evil took about as good a crack at it as any, but even then, it gets pretty fuzzy when there's a large group of zombies that should have to eat, but don't want to eat each other. They had the virus re-energizing the cells, which in the short term, is fine.
With TWD, you've got the long time frame, and even seasonal changes to deal with. Starts getting tougher to ignore those sorts of things in the long term. Maybe they can go a month without food by burning through every reserve in the body, but a year later? You show the zombies decomposing a bit, but in the Gerogia summer heat, with the decomposing gases and no way to cool? BLAM!
I appreciate what we've been getting (minus the season long 'search' for the girl on the farm that just DRAGGED), but I wouldn't mind more mythology on this one. I ENJOYED the CDC part. For the most part, we've stayed with Rick's perspective, so we don't know much of anything about what happened, how it happened, if there's anyone anywhere able to do anything to help. We woke up in Zombieland, fully established. I'd like at least some more bits and pieces filled in at some point, even if we don't get the full story.
Things like warp drive are easier to gloss over, it's basically just magic because you don't know anything about any of the components (and believed to be physically impossible, give or take, so you can just push the 'i believe' button and move on). With this, you know what the human body is made of, how it reacts to various conditions, what it takes to fuel it, heal it, rest it, etc. It's a tougher leap of faith, because it isn't asking us to believe in magic, it's asking us to ignore things we actually KNOW stuff about.
It's cool if you show zombies having to eat the flesh of the living to gain energy and survive. And maybe there's something there that they can't get from other dead people, which is why they don't eat each other. But then you gotta show them eating, can't leave them dormant, or wandering around a pharmacy for a year without consequences. Can't bake them in the sun without cooling off, sweating, or drinking water/blood, and then not have them looking like a squishy mummy later on.
I'm enjoying the show a lot, and know they're stuck with certain premises that they're playing to, just saying it's harder to suspend disbelief when the subject is something we know more about. Zombies are a great horror subject (and why the Borg were so great when they came out originally), but just don't hold up to a long-term project without more explaination or eye-rolling disbelief.