Because it is respectful to do so, and if you refuse to call someone by the gender they consider themselves or prefer, because "biologically" they "aren't", to some degree or other, you're just being an edgelord.
There is no gain in refusing to acknowledge that binary gender is a social construct and we actually don't need to think of gender as a binary at all.
People that refuse to accept that are just scared of something that doesn't make sense to them, imo.
I guess I consider transwomen women because to me, they are. I don't think of gender as binary or "penis or vagina", i consider brain wiring, fetal development, disposition in a societal context, brain chemistry, gender disphoria, intersex conditions and unknowns.
Gender could be a spectrum, it almost certainly is.
We made up the binary, it's not actually reflective of reality.