Why can’t I see my iPhone battery percentage on my Apple Watch?

You can get some of what you want on your watch with Battery Life.

You can make it a complication on some watch faces.

M1 Mac mini, macOS 15.5;  Watch, series 10 GPS-only, watchOS 11.5; iPhone 16, iOS 18.5; TV 4th Gen, tvOS 18.5 HomePods, audioOS 18.5

not that I know of

but you can suggest they add the feature using

Feedback - Watch - Apple

a reason they might have omitted adding it, could be that if one has the cellular model and or the gps only and was not within range of the iphone, so it could not have any knowledge about the iphones batt level then some users might be confused and flood their support channels with complaints, because they expected that such info could be transferred over the internet or something like that