DISCLAIMER: My only knowledge about fate is the Fate/Zero and Unlimited Blade Works anime adaptations, as well as the Fate/Extella video game. This is something I'm genuinely curious about.
The Fate franchise is all about summoning Heroic Spirits to participate in the Holy Grail War. It is filled with fictional depictions of historical and mythical figures, such as Hercules, Achilles, King Arthur, Alexander the Great, and more.
There are many instances of turning male historical figures into female Heroic Spirits, including but not limited to King Arthur, Nobunaga Oda, Musashi Miyamoto, Francis Drake, and many more.
However, the reverse doesn't happen. Female historical figures stay female when summoned as Heroic Spirits and aren't turned into males.
Why is this? Is it for artistic reasons? Story-telling reasons?