You are overlooking that humanity is subjective. I am human but that thing with a different skin color isnt.

So i can say "All Humans are equal and free and bla" and simultaniously enslave that differing colored creature over there.

Eventually someone who hast not shit for brains comes to Power and sees that the enslaved are just differing colored humans and have the motivation to truly make "All Humans are equal and free and bla"

Now since History exists some people are more "equal and free and bla" since some people just became Humans yesterday and need to some correcting Treatment so they can be "equal and free and bla" just as much as the other Humans. And correcting means giving them the same rights and bla.

But for some reason some people think that If some other group gain some rights (that they themselves already have) that they deserve a participation trophy as well.