[Cryptography] WhatsApp: Why asymmetric key instead of symmetric keys?

[Ismail Kizir]

>PSK authentication would only cover the case where users want to exchange keys in person, which in practice has been shown to be a very small number of people. WhatsApp is targeting over >a billion users: asking a billion people to do an in-person PSK exchange with all of their contacts is a nonstarter.

Thank you for your quick Answer Tony,

But I meant, "why not both"?
It's not difficult to design a protocol which also supports pre shared keys.