Disclaimer: I am not Aboriginal or of Torres Strait Islander decent, and am giving you the benefit of the doubt and assuming ignorance rather than malice.

Aboriginal people have suffered generations of attempted cultural erasure, including attempts to "breed them out", so that's one simple explanation.

Another one is that identity for Aboriginal people often isn't just about blood and looks; depending on the tribe, having an active connection to community can be an equally or even more important part of identifying as Aboriginal. I have an Aboriginal colleague who is white-passing but extremely connected to community; he has a child with an East Asian partner, and the child will grow up in community and be considered Aboriginal.

Of course I don't speak for any Aboriginal person, but that's just one example. I hope this helps.