Why can't I add a Homeassistant scene to Homekit as a scene?

It would be excellent if Homeassistant could add a scene to Homekit as a Homekit scene, similar to how a Philips Hue scene is added to Homekit. This results in the scene being easily activated by voice, and would of course appear as a scene inside Homekit rather than a switch, which is not what a user would expect to see.

Would be great, but is that not just something that could be added in HomeKit Controller?

Yes but saves having to maintain two separate scene lists and HA can become the primary scene creator.