Why do I need to be connected to WiFi to back up my iPhone?

Backing up your iPhone will use incredibly large amounts of cellular data. Apple, in an effort to save your data, doesn't allow you to just back up without Wi-Fi or iTunes. Read more here: How to back up your iPhone, iPad, and iPod touch - Apple Support

We cannot answer "why" questions here. Apple applies policies, and they are whatever they are. They may or may not take actions in the future.

As you have observed, Wi-Fi is required for iCloud backups.

You may provide feedback if you wish at:

Feedback - iPhone - Apple

Pleaseanswerme1 wrote:

Thank you for your answer but, I have unlimited data which is very common, so why can’t I just that if I’m willing to use my data.

Part of the issue is that the cellular service providers don't necessarily like that you're willing to use your cellular data for backups, and Apple listens to them. I believe there's also a limit to the size of apps that can be downloaded via cellular.