Why can’t you update it over mobile data but only on WiFi

If you have an iPhone 5G model, and are on a 5G network that allows it, you can update your 5G iPhone over 5G cellular. All non-5G iPhone models however can only update over WiFi.

See -> Use 5G with your iPhone - Apple Support

The cell carriers put this requirement in place to keep from overloading their networks before 5G came along, Apple is only following their rules.