It's (relatively) cheap for Netflix or their CDN partners to connect their content-serving systems to a few dozen "meet me" rooms where it's "easy" for Comcast to hook as many wires up as necessary to receive the requested data into their network at an acceptable rate. That's all Netflix or their CDNs need to worry about. And so it's (relatively) cheap for them to scale the amount of data they serve as their customer base, average viewing time and stream quality grow.
But Comcast has to deliver all this data to tens of millions of customers spread out around the nation. While it would be nice if their networks were sufficiently provisioned to serve, say, half their customers 20Mbit/sec continuously during primetime, that is just not the network they have today. It will always be cheaper for Netflix to turn up the spigot than for Comcast to build out its infrastructure. Should "network neutrality" force Comcast to spend billions every time Netflix doubles its streaming rate?