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
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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.