Why can’t I offload messages to iCloud like I can photos and other data?

iMessage is taking up a large percentage of data on my phone and I was looking for a way to free up that storage with iCloud. I’ve gathered from other threads that it isn’t possible, iCloud backed up messages have to be stored on your device and if you delete message data on your phone it’s deleted from iCloud on all of your devices. I am just wondering if there is a reason this is the case and a feature couldn’t be implemented to do this like it has been for photos?