Well I'm a bushie from way back, and out here in the boonies, well it's just one of those things you do. Mind you when I go into the big smoke (next week actually) well of course you're not going to wave at every car you pass. However, I still give a small wave when somebody gives me my right of way. Yeah I know that they are supposed to give way, but it is just a common courtesy to say thanks. If you don't acknowledge that a driver has done the right thing, then maybe down the track they won't do the right thing.
But you're absolutely correct when you say the type of car you're driving has a lot to do with it. Out here, if I'm in my ute, virtually everyone waves, well at least the locals do. We get a lot of tourists out here and some wave and some don't.
Thirty years back when we were doing a lot of outback touring, you always gave a wave. In fact on some rarely used outback tracks, if you came across another vehicle, you actually pulled over and had a chat. And I even waved at the wallopers when they went passed, and mostly I got a wave back. Still wave a the local copper, but then I know him anyway. Even did some interstate trucking way back, and you just did not pass another truck without giving a wave, or at night time, a flick of your indicators.
To me, i guess it's just an Aussie thing. We're a friendly mob and I just wish to hell that all these new arrivals would take up our ways. Just like after WW11, we couldn't speak their lingo, but you always said g'day. And those immigrants ended up becoming part of the backbone of the country.
Cheers mates