Friction. Getting a second phone number requires you to actually spend time and money on it. And with services blocking "prepaid" phone numbers (as mentioned above) you need either multiple phones or constantly swapping cards. Most people don't want to go through the hassle.

There is nothing that is absolutely categorically capable of singularly identifying an individual natural person except that person’s physical body and even then biometrics need to be carefully created.

Everything else has a statistical probability of correlating with a natural person. The probability of phone numbers as identification is good enough. In fact it is great.

All or nothing reasoning is incorrect. It is only useful when you need to capture literally every case, which isn’t the problem here.