Politics is part of everyday life regardless of whether you care or not and I’m not talking about the media circus that is the US election machine but rather about the cost of products and services, prevention of crime and what is determined as a crime, the rules of how to conduct business, provision of services like roads, water, sewage by government/authorities and the rates of taxes that fund those services to make but a few. If you don’t care about politics them I would imagine you have no opinions on anything of that nature and would never complain of your cost of living increased or you couldn’t access medical treatment or call a fire station in a time of need or about who had the right to vote?
Making voting mandatory in Australia also protects people who want to vote from being exploited by their employers. Imagine the scenario where there is a referendum on a matter that would be very beneficial to businesses but exploits employees. A corrupt business may decide to schedule their employees to work a 12 hour shift that would prevent them from being able to vote if the business knew it’s employees would vote against the legislation when they want it to pass.
You don’t actually have to cast your vote in Australia, just show up but even if it was mandatory to cast it, they can always be spoiled. You could even write “I have no opinion on politics and don’t want to vote for anyone”.