My understanding is that it's a regulatory requirement, the Govt wants to know who specifically owns what SIM. If I remember right, this was introduced post 9/11. I may be spectacularly wrong on both fronts, though!
That doesn’t stop these companies just going for the big ID though, and for postpaid they are offering credit and will definitely require 100 points of ID.
The ACMA determination is pretty hard to parse unfortunately.
Then they detained an innocent person for the longest time without charge in recent history, including solitary for 23 hours per day, and not letting him access legal advice or contact his family, because terrorism.
https://www.comparitech.com/blog/vpn-privacy/sim-card-regist...
US Senate tried to pass a similar law back in 2010. Bill sponsors were Chuck Schumer and John Cornyn, in case you ever see those names on your ballot.