they do, you log in based on your number so they have to store it one way or another

This sounds amazing and all but in most parts of the world a) you don’t have exclusively mobile internet SIMs b) you don’t have eSIMs and eSIM compatible devices c) mobile connections are metered (and expensive), so people talk on the ordinary cellular phone connections d) mobile data is slow and laggy, so calls frequently drop/are choppy/don’t connect at all e) you don’t have wifi in a lot of places f) people aren’t necessarily familiar with VOIP services