Why do I have to have Wi-Fi?

See the following from Back up iPhone - Apple Support

Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular.

BILLHEREISNOTHERE wrote:

What if I don’t want to use Wi-Fi

IF you're referring to backup then wifi is required to backup to the lcoud, otherwise use itunes.

I’m sorry if your having an issue with your device but you have the option to have Wi-Fi turned off, as in all models of iPhones the Wi-Fi function is on all software updates

But if you do turn it off you can’t back up to iCloud, which is what the OP was asking.