I'm assuming you're talking about 90% of zombie movies don't tell why. I'd say that most movies do indeed tell you why things happen and how because it's naturally what the mind wants to know when being told a story.

Personally, I want to know. People can look for loopholes all they want, I'm pretty certain we've only found one or two conflicts in the story and both were pretty minor (the one about the hummer hatch in the beginning, and there was another one I don't recall right now....) I believe if we were to be told how this happened, it wouldn't take away from the story, and I'm positive Kc and Company would be able to reveal it so we won't be able to find loopholes (and if there are loopholes, he's got a fanatic fanbase that will defend the loopholes)