Tieing 2fa directly to the phone is much better than tieing it to a phone number, until you don't control your phone. Then, how do you recover? You can't go down to the local Google (or whoever) kiosk, prove your identity, and get a new device added to 2fa, but you can get a new sim (downside, so can pretty much any carrier kiosk employee, or social engineer).

Also, phone numbers are pretty useful for contacting people, and most people have an address book with their friends and family and other important contacts. There's tremendous social value for people in those numbers continuing to work, to the extent possible. You're never going to be able to tell everyone who might want to know when your number changes, especially if you for a new number when yours was assigned to someone else.