Because type correctness does not imply branch correctness. SQL has side effects of interpretation, and any string/query builder that is not aware of grammatical implications should be avoided in my opinion.
I clearly don't understand something about implications. Could you please elaborate or give a link to read about it? What is branch correctness? How could it be exploited? How does sanitization prevent it?
sqlx looks like a usual builder, I don't see nothing criminal about it.
Why do you need a sanitization for bind parameters?