Let me start with a caveat -- I have not seen every zombie movie ever. Please do not point out how in Frederico LaDouchey's 1958 Italian epic Il Zomboner all of these questions were answered before the credits finished rolling. I'm just saying that, by and large, zombie fiction refuses to acknowledge these small points that, when you think about them, are kind of a big deal for the fantasy to play out.
That said, I watch a lot of zombie movies. More than maybe is normal, and once I get to the third or so watching of a particular movie, I'll stop and wonder about these details and why it is that no one wants to explain them to me. Then I get sad that I'm demanding answers from a piece of fiction, so I eat some ice cream and fall asleep in my comfy chair, and when Gladstone wakes me up the next morning, he tells me everything will be OK. But secretly I still want to know.