Traditional doorbell needs an electrically powered button on the exterior.
Phone needs visitor to have your private contact info or you would need to publicly add a note with that to your door.
Also, a real traditional doorbell is mechanically coupled to the bell, with no electricity at all.
What reason would you want to be notified of unknown visitors knocking on your doors when you're away? The only people I can see fitting this description are delivery drivers & salesmen?
Further, what if you're home and your phone is in another room, is on silent, or is dead?
What is you have a spouse or housemate? Do both of you get notified?
And there's also a subset of people who get frustrated by having to use QR codes in restaurants, who I suspect would be equally frustrated by this.