We have a small population and mandatory voting means everyone gets a vote by default. We also have a different culture around voting because the majority of us have to do it. We have sausage sizzles and democracy dogs. I've personally worked at polls all over my state and there's never been a line longer than 10 people. It takes most people like 5 mins max to vote. We make voting easy in Australia because everyone has to do it.

It's worth noting that it's not all that mandatory. It's relatively easy to simply avoid enrolling to vote. You're not automatically enrolled in other words. Also it's really easy to just sign your name off at the poll and hand in a blank vote. The worst outcome of not voting is a fine that you can pretty easily get out of as well.

Is Belgium also a surprising exception?

I think this is the wrong question. Why is voting optional? We can crow about nebulous "freedom", but we are forced to do much harder things all the time. Once every few years, we're asked to make a few decisions.

If the idea is that the citizens select leaders, but it's incredibly easy to opt out, what you have is a biased selection committee. We can argue all day about the various biases involved and if they matter, but the reality is that it's not burdensome on Australians and it actually makes sense to get your source election data as close to based on public sentiment as you can.