Why can’t I back up my phone with my cellular data
Why can’t I use my cellular data to back up my iPhone and update my iOS
Question marked as
Because that isn't the way things are set up. Both require WiFi.
Backup methods for iPhone, iPad, and iPod touch - Apple Support
Update your iPhone or iPad - Apple Support
If you want to, you can send Apple feedback. They won't answer, but at least will know there is a problem or a suggestion for change. If enough people send feedback, it may get the problem/suggested change solved sooner. Pick the closest topic you can find.