This would also be impossible if services stopped demanding your phone number to make an account.
This is a growing trend in consumer services, and it's a privacy nightmare.
Imagine if they demanded your SSN to sign up? A phone number is no different or less sensitive a unique identifier, perhaps even moreso these days.
There are widespread reports of delivery businesses selling their phone number databases (with associated credit card suffixes, delivery addresses, order history, et c) to large advertising companies for data mining.
Providing your direct cell number to an app is basically like providing your home address and a bunch of other sensitive data. Don't do it, or make a burner gmail account to get a disposable Google Voice number for each account that you must have that demands a phone number. Then, that number isn't reused and an attacker that obtains your mobile number can't attack your login method for other apps.
Reusing phone numbers is about as bad as reusing passwords.