Although, it isn't Australia's birthday. We weren't a country when Phillip landed. What us recent migrants think of 'Australia' didn't really come about until Federation when the different colonies got joined under one government. Before that we were a bunch of colonies bickering over supplies and prestige, a legacy that still lasts in a lot of ways. I guess it's celebrated because for as long as most of us can remember, that's the way it's been. And a smart politician never fucks with tradition.