Tapping out words at people does not equal social interaction -- at least not as far as our brains are concerned.
Objectively, people today have fewer close friends, with whom they spend less time. A significant minority -- 10 to 15%, depending where you look -- don't have a single person with whom they share any real intimacy (emotional or physical). Not even a close friend. Such high levels of complete interpersonal isolation are unheard of in Western history (and the West has never been the most social society to begin with). They may have a wide array of acquaintances and people they connect with through technology, but they are not getting adequate levels of deeper interaction.
Putting out a Tweet about an awesome band you found is not a replacement for sharing your fears, or a nice hug. But people today are often replacing the latter with the former, and as a result of this, people are reporting themselves to be much more lonely than they did in the past.
As to why, I'm sure that's complex, but technology itself might have a lot to do with it. It's easy to use technology as a crutch to avoid the things many of us find anxiety-inducing about interacting, especially at more intimate levels: risk of rejection, saying the wrong thing, not looking our best -- technology can allow us to avoid all of those. And so we do.
The end result is less meaningful interaction, more loneliness, and some studies show also more anxiety (avoiding a scary thing tends to deepen the fear).
Like I said, I don't see this lasting indefinitely -- humans simply cannot survive like that, psychologically speaking. But it's a very real problem at this point in history, and without conversation, it won't correct.
Dating expectations and that whole mess is a different, although perhaps tangentially related, issue.