Because it's matter of respect.

You don't go out in public saying "Your work is not good because you made this character a woman, so now I'm saying she's Trannie so it means it's better and I have the authority to do so because I say so".

Is like screaming at Shakespeare about how offensive s Romeo and Juliet because he didn't make Romeo gay, or dissing Da Vinci because he painted the Mona Lisa with an Adam Apple.

And before you say something about how I can't compare those kind of art to videogames, I can because both are some kind of craft media.

Finally, if this was a thing like "This is my headcanon" , then everybody would totally be fine!. Everyone have their own headcannon or their stories or characters.
But the moment you say "My headcanon is the real deal because it's much better than the one the creator wrote" is just crossing the line

A work of fiction….made by someone who write the story, characters and places. If he says Samus is a woman….

she's a freaking woman