This feature of whatsapp looks like a bad practice refering to the privacy of their users, I guess it has received critics about that and even though it's nowadays an old application it continues the same.
I think that something like relating an username to a certain phone number has to be easy, even trivial, sort of how a DNS works but whatsapp still stays the same, so my guess might be wrong.
Is there any reason that it is done this way?