For data collection purposes, a "sex" field is more informative than a "pronoun" field. That's not to say a "pronoun" field wouldn't be informative, only that pronouns supervene on sex except in outlier cases.
Of course the arrangement of my internal organs is relevant to doctors and medical sciences. As is my blood type and my cholesterol level. That doesn't mean you should be asking for my blood type when I'm registering for an online service (unless, of course, that service involves blood donation or something). That's the whole point -- it's really, genuinely, just not relevant to the vast majority of cases where it's asked.