The answer to this is fairly obvious if you would avoid having a knee-jerk reaction: Because the dimension of gender inflects grammar, even in "ungendered" languages like English, but even moreso in most Romance or Germanic languages.
This is literally the reason many signup forms ask for gender - to know which pronouns to use. Why not just ask that? In languages which have other inflections, those fields are also often asked for - I've worked on software that needed to support Korean honorifics, and even in German you get dropdowns for "Dr." vs. "Prof. Dr.", "Prof. Dr. med.", etc.
German language doesn't even have different words for sex and gender, they are the same. It's just Americans pushing their latest group think on rest of they world. The whole pronoun issue is absurd and invented problem.
This is literally the reason many signup forms ask for gender - to know which pronouns to use. Why not just ask that? In languages which have other inflections, those fields are also often asked for - I've worked on software that needed to support Korean honorifics, and even in German you get dropdowns for "Dr." vs. "Prof. Dr.", "Prof. Dr. med.", etc.