Seriously mate?
You really ask why? Well, maybe because a real person, with real feelings, with a real voice, with real things to tell you, a human being in other words is actually trying to reach you while you, in the first place, were just browsing instagram/Facebook. As if just being 100% with that person on the phone wasn't good enough. Given being on the phone doesn't match seeing each other, now people want a compact phone app so they can be even less present to the conversation.
No personal offense really, but when I read your answer I thought that this was some Black Mirror shiiiite.