I mean unless there’s an Apple employee willing to speak up we’ll never have a “true” reason but given that this is how it’s always been since wireless CarPlay was a thing, that’s the most logical reasoning. You’re not draining battery when plugged in but you are when you aren’t, and the expectation is that you’re also not relying on an indicator in CarPlay to know your phone is fully charged like you would when normally charging.Still doesn;t explain why the battery icons shows on your phone when charging it via a cable (in the car or out) then.
Why one situation for CarPlay and different for iPhone and other devices?
You can make the argument CarPlay is displaying a different UI which is why a battery indicator isn’t permanently visible the way it would be normally. To my knowledge I don’t think there’s any other facet of the system that behaves like CarPlay so it’s not like there are any set rules for it to follow.