Why does the iPhone let you know when your Apple Watch is fully charged but the Apple Watch doesn’t do the same when your iPhone is fully charged?

Makes perfect sense. In order for that to occur the iPhone needs to push battery status information to the watch. This I assume has not been done, not only that, I assume there is no application on the watch to obtain this information.

I suspect there may be third party applications that can do this.

Probably not that high of a priority, or something nobody has considered.