Why do some people dispute the fact that Aborigines are not the first Australians?

If you say that they are the first humans here, then that is fine (although many believe that even that is not the case), but they were not the first things here. Crocodiles, kangaroos, numbats, wombats, thylacines, quolls and tasmanian devils have been here for over 4 million years. Crocodiles 200 million years.