In Australia, you place your carry on luggage onto a tray and it passes through an xray machine, at the same time, you walk through a metal detector. Takes about 30 seconds depending on the line.

It still feels incongruent with the reality of the situation in my opinion. I can hop on a bus with 200 other people, or on a train with literally 0 security carrying whatever I want in a bag with no staff nearby either.

Not to the same extent though - for example I can't remember if I ever had to take my shoes off (maybe there was a couple of months where we had to do it back after the attempt happened in December 2021?), so I was pretty shocked to go to the US for a work trip in 2019 and have to do that. Here in Australia there's no liquid limit in carry on for domestic flights.

Isn't this because there otherwise wouldn't be allowed to fly into US airspace?

It's not just security theater. It shifts the attack vector entirely. Instead of airplanes as weapons that could be used to kill thousands, terrorists can blow up a few hundred people.

Those checkpoints are only there to provide a soft target instead of letting it be a plane.