Same reason as why dictionaries have multiple definitions for a single word. The specific meaning of a word is dependent on the context it’s used in. Languages also evolve as we discover more about the world we live in – new words are created and old ones get updated to better fit reality.

Sometimes people willfully ignore that in an attempt to force a discussion to a conclusion that isn’t true. They don’t like nuance or context, because those are the death of a bad argument.

…because that’s how language works?

If someone shows me a person and asks if that person is a woman, it will depend on “whether i perceive them as a woman in that moment”

If we’re talking about gender as a social concept in society, a woman is “a person the majority perceives as a woman”

If we’re talking about one’s identity, a woman is simply “someone who perceives themselves as a woman”

If we’re talking about genetics, a woman is a person with 2 x chromosomes, if we’re talking about primary sex characteristics, a woman is a persob with a vagina (or do we wanna limit it to having had one from birth? Both yes and no answers might be relevant depending on the context), etc.

even if you take gender out of the picture entirely and pretend like someone who looks like a woman, sounds like a woman, behaves like a woman somehow isn’t a woman, you’d still need context to define it.

It’s also a question that I have never fucking asked myself in my life in a general sense, because it’s utterly irrelevant except in very specific contexts.

A woman is a mature female human.

A girl is a young female human.