iCloud backup

Because that’s the way that Apple designed it to work. A WiFi connection would be more stable than cellular in many instances and the data use might be prohibitive after a while.

You can backup using iTunes on a computer that has an internet connection via an Ethernet cable. You don’t need WiFi to use iTunes this way.

FWIW, I backup to iCloud and to it tunes on my Mac. I have been doing this since day one when the ability to backup over WiFi was introduced.

You don't if you back it up on your computer. I would always do so, and never back a phone up on iCloud. Please see this Apple document that covers backing up to a computer.

About backups for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT204136