I'll tell you why. It's because our TV networks are saturated with American shows and films. American culture is rammed down our throats too and it has been for decades.
It's a matter of exposure. We know what Americans sound like so it makes it easier to imitate their accents.
Also, we're all just super talented and can do
anything
"Puss rules!"
"It's Pus"

For some reason, actors and actresses from Australia (and probably NZ) can do a credible "American accent" better than performers from any other nation. Even Canadians! I don't know why.
E pluribus unum

Seems like just a coincidence. The director wanted relatively unknown actors to play the leads and wanted Crowe because of his performance in Romper Stomper. Pierce auditioned.
Hail to the king, baby.

Guy Pearce was born in UK.

Has to be cold? Let me guess, you're British
I'm an Aussie and I hate Foster's. So does everyone I know. It's not a popular beer by any means.
Cooper's Sparkling Ale or XXXX for me, please.

No-one in Australia drinks Foster's, or at least no-one I've ever met. In South Australia we'll do anything for a glass of Shiraz.

They'll work for Vegemite Sandwiches, and a can of Fosters (Has to be cold though).

Yes, because the fact that he has lived in Australia most of his life, owns an Australian Football club, married an Australian, has Australian children, was even on an Australian stamp and is identified by most (including himself) as an Aussie isn't enough evidence, Wikipedia must have been edited.

Anyone can edit Wiki.

From Wikipedia:
Russell Ira Crowe (born 7 April 1964) is an actor, film producer and musician. Although a New Zealand citizen, he has lived most of his life in Australia and identifies himself as an Australian.

Russell Crowe is a Kiwi not an Australian