world interest rates

Could someone please explain to me the following in layman's terms.This has been doing my head in for ages but I just can't quite get it.
The OZ banks get 30-40% of their funding O/S(presumably the US mainly) which is why they have had to do a little "value adding" to the RB hikes?Right?
However all I seem to hear is that the US Fed is printing money like there is going to be a shortage of trees and US interest rates are at all time lows(near zero).
My dumb questions are;
Why is the premium being charged when US rates are so much lower than ours?
Why don't the big four borrow ALL their funds from the US et al?
Please try not to be too condescending with the answer as I am sure there are many,many out there whom have been wondering the same thing.

Thankyou all in advance.