Why is setting the default theme so well hidden?

Why is setting a default theme something that we set on startup via a service triggered by an automation instead of a configuration setting we set via frontend: in yaml or via the UI?

This doesn’t answer your question, but be aware that you can change the active theme in the UI under the Profile settings. Maybe the logic is that most users will rarely see the default theme if they choose to set their own theme, so there isn’t a huge demand to have UI control?