To be honest, you could also say that Valve keeps rehashing CS and L4D, and no one really cares too much (aside from people who still think the Half-Life series isn't dead). And honestly, it's not completely bad. It renews interest in series and feeds the fanbase. Just think about if Valve made a TF3 that had competitive support, but in other respects was not much different from TF2 aside from better weapon balancing. We would see it as a revolution (not sure if this is the word I was looking for, I need to get to bed), but non-TF2 players looking at TF3 (and probably even many pub TF2 players) would see it as a rehash of TF2. My point is that just because there doesn't appear to be any major changes on the outside doesn't mean there are none.
If this doesn't make any sense it's because I'm tired.