Australia has both compulsory voting, like Peru, and a separate electoral roll that you have to (and must) apply to get on, like the US. It's not a US thing, it's a common law anglosphere thing.
But I'll give you the benefit of the doubt and assume it's actually easy in Australia. That still doesn't really explain why it's necessary.
For people not on the roll, there is a special secondary method of voting called a declaration vote that allows you e.g. to vote even if you haven't enrolled. But this process wouldn't scale very well - these votes aren't counted for days after as they need to check the details you declared - and is intended as a safety net.
This seems a bit like Chesterton's fence...