Because there are very few programs that can drive subscribership on their own. Just like there is a benefit to pool resources for the creation of assets, like a network that can produce and distribute the content, there is a benefit to pooling potential subscriber bases to establish a steady, predictable revenue stream.
The content is not going away, the distribution method is changing. Belts will probably need to tighten but perhaps we are underestimating the potential value of the alternate distribution methods.