Technically it's because the current system can only support 4.3bn machine addressable numbers, the last of which is soon to be allocated. But the simple explanation is this: the internet's full. In a year's time it could even be rendered useless. How did people do things before the internet? For the young, a primer:
Social networking. In the past people engaged socially mostly by prior arrangement, using the now-defunct RSVP protocol. One's offline presence was visible to anyone, so you had to constantly check to make sure you weren't going outside in your pyjamas.
Gossiping. Malicious gossip was conducted orally over a peer-to-peer network. It was slow, and information could become distorted along the way. On the plus side, you weren't limited to 140 characters, and no one ever checked to see if you might be lying.
Stealing music. Connect cassette player to radio, wait for good song, press "record".
Finding and buying stuff. All shops had phone numbers, gathered in "directories". Next time someone drops one of those yellow-paged composting blocks at your door, look inside.
Making several hundred holiday photos available for viewing by friends. If I told you how we used to do this, you wouldn't believe me.
