Why is the exact model of iPhones not available?

In the "models" analysis, you will certainly find a very large proportion of the models attributed to iPhone, without more details on the exact model (iPhone 11, iPhone XR, etc.)

We display "iPhone" when the precise model cannot be identified from the User Agent sent by Apple. Their UAs are, unfortunately, more and more generic, and don't allow us to determine the exact model.

Here's a UA sent by Apple on an event that has "Iphone" as the Device - Technical Name, without information to differentiate the model:

"Mozilla/5.0 (iPhone; CPU iPhone OS 15_3_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.3 Mobile/15E148 Safari/604.1"