I'd say most people react in that way because they're constantly bombarded with the same questions over and over. Usually the question is very simple and there are multiple topics on the site dealing with the same issue but slightly reworded. You even see it in large topics on this site, the same question repeated when it was asked a page or two ago. Most of the time these things can be found in less time by seaching google or even searching the forum in question. I'd say it's mostly the simplicity of the question and the fact it has been answered countless times that annoys people. More complex questions or ones that have some amount of thought put into them that can't be answered with a couple of words are looked on more favourably.
You may have a point there, but there are times when I do try to just "google it", the search result brings me to forums of people asking the exact same question with the response of "googling it". Perhaps, that is just a sign that google's search engine isn't as great as we all think it to be, afterall what good is a result that is almost a loop onto itself? So, if the answer was indeed somewhere on the web, and google had instead buried it under dead end forum posts of people arguing(because for some reason they get to be on the front pages of that search) Wouldn't the suggestion to just "google it" not be valid at all?