"Nothing prevents you to broadcast a number of channels"

That's not entirely true. If you were to simply try and broadcast multicast IP traffic, it would get killed pretty quickly by various routers throughout the internet. Then if you tried to get around that through IP tunnelling, you'd run into a whole host of problems depending on your situation. If you're HBO and you're sending video directly to people, your internet connection provider is going to charge you a hell of a lot of money, and the ISP's of your subscribers are going to be bitching up a storm.  

The simplest answer is just that the backbone of the internet is not fast enough to send broadcast video.