What could you use it for?
When I started working, more experienced colleagues always warned to never guess gender, and always let people enter their gender, because slight mistakes already lead to highly offended customers/users/whatever.
This was before the whole non-binary movement, and as a result, by 2005 that already changed to "let people enter how they want to be addressed", since that was what most gender fields were used for anyway.
So given that a small mistake will cause an enormous backlash, what can you actually use it for? Statistics maybe, but in most cases those will be anonymized, so you won't have the name. So that leaves dodgy webcrawling to send spam.