Why do US/Canadian universities make you do master even if you have a first class honours from Australia?
The principal difference between degrees in the US and Canada and those obtained in other countries is that the bachelor's degree in the US typically contains fewer "technical" courses than a comparable bachelor's degree from other countries. For instance, a student in economics may only have twelve courses total across four years that are economics courses; the typical international student may have twice as many. This is because most US and Canadian schools incorporate a "general studies" requirement into their degrees, which reduces the amount of credits available for the major.

Consequently, the coursework phase of the PhD program is to bring everybody up to the same level—also a necessity, given the diversity of undergraduate curricula—before moving on to the research phase.

Also note that this is not a universal requirement. Some programs will admit directly to the PhD with only a four-year undergraduate degree, with no expectations or requirements of acquiring a master's degree. (This is my case—I have no master's.)