Because of all the spambots most users have their whisper setting set to block incoming whispers. If 2 users have whispers blocked they cant whisper each other at all, even if they know and trust eachother, without having to activate whispers and deactivate it again afterwards.

If the friend list feature that used to exist would be brought back then they could keep whispers on blocked for non-friends while still being able to whisper with friends.

It is always a MASSIVE nuisance when i for example want to share a youtube link with someone but they have to unblock whispers first (because in many channel chats links get filtered out or even give you a timeout).

At this point i also want to emphasize that i never understood why the friend feature was removed and always thought that was a dumb decision.