Oaths of the Mountains and Seas, A Pact with the Immortal Realm

Why Aren't There Any Palace-Intrigue Stories Where the Heroine Starts Out as the Empress?

In every palace-intrigue novel I read, the heroine enters the palace through the selection, then schemes against all the consorts and the empress, and eventually her son becomes the next emperor while she becomes the empress dowager. I've never seen a heroine start out as empress and then rise to become empress dowager.