I think you may find some enlightenment in this older answer of mine: It used to be acceptable to dress young boys in dresses. Nowadays dresses are considered "feminine," and dressing a young boy in a dress would be cause for some side-eye. Why did this change?

The issue of exactly "why" is not quite answerable - the Victorians did it because previous generations did it. Rather than dressing them "as girls", this was simply the norm for dressing children before it became important to differentiate them by gender. In the late twentieth century, dressing children "unisex" came to mean dressing girls as boys; this is just the opposite viewpoint.