why i cant use mobile data to update the ios 14 ?

I am going to offer a slightly different response and a workaround.

I have a 100GB per month cellular contract and small wifi allowance, and so I have the same dilemma. The limit is Apple policy at the request of the cellular companies, and needs reviewing as 5G draws near. In my own opinion it makes no sense when the provider is offering a huge bandwidth like that.

If you have a Mac or PC with iTunes that you have used to update your phone successfully, you can tether the computer to the phone's personal hotspot and connect to the internet.

Then using iTunes (Finder in Catalina) you can initiate the update, and after the download has finished (about 4.2GB for an iPhone 7) the phone will be prepared for update, go black, and the tethering will terminate. However an internet connection is not required for the rest of the update which will continue until the phone comes back to life.

I have done this three times with no problems, if it is of any help to you.

Obviously you can always update one phone or iPad using another phone's personal hotspot if you have a generous cellular allowance of 50 to 100 GBs.

Because updating requires using Wifi or connecting to your home computer and doing the update via iTunes.

The cellular providers do NOT permit downloading that much data over cellular.

I should add the tethering using Mac is via USB, and to update one phone by tethering to another phone is by WiFi, i.e. OTA update