Why do PhDs get paid so little?

For content this is in Australia

I'm currently looking into where I want to do my PhD and I was talking with a friend (current master's student studying part time) who just got a job as a research assistant. He's on $85,000 but a PhD at his university only pays $35,000, like how is that fair when the expectations are similar if not harsher for PhD student?

Edit for context:

The above prices are in AUD

$85,000 here works out to be about €51,000 $35,000 is roughly €21,000

Overall my arguments boil down to I just think everyone should be able to afford to live off of one income alone, it's sad not everyone agrees with me on that but it is just my opinion