Because usernames and passwords can be implemented without reliance on 3rd party infrastructure and they are culturally accepted and understood - however poorly

Because usernames and passwords can be implemented without reliance on 3rd party infrastructure and they are culturally accepted and understood - however poorly - by everyone. We're still using 80 character lines in programming because of IBM punch card design from 1928. Similarly, I doubt the password will completely go away within our grandchildren's lifetimes. That said, our mobile devices are likely to play an increasing role in authentication for everyday activities.