Simple, we get mad when people revive a topic with a post that doesn't relate to the topic at hand, has no content, is something along the lines of "I agree." to a topic that was dead and buried, ect.
If you really had something to contribute, reviving an old thread is the better way to go because you can read all the responses before hand and see the general direction in which the responses will go.
Basically, when you've been around as long as some of us here, you get tired of seeing the same idea (in basically the same words) created time after time after time after time.
Get it now or should I keep typing? =)