As a cable TV provider, Comcast is supposed to be a regulated monopoly. And there's a reason for that, and it's the physical layer.
In most areas, you have one electric company, one phone company, and one cable TV company. That's because each provider has to run their own wires, and a hundred years ago, city leaders didn't want to have all the separate wires running everywhere if each electric company, for example, had to have their own power lines to serve their own customers. To avoid this, they granted a monopoly to a single company, but it's a regulated monopoly -- the rates they charge to customers are controlled by the government; if they want to raise rates, they have to ask for and get approval.
Nowadays, things are a little different in that there are sometimes requirements for the company that built and owns the physical layer to allow other companies to provide end-user service. But since the company that owns the physical layer competes for end users with companies that rely on them to provide the service, there's often not a financial advantage to the end user to use a different company. Regulation would have to force the owner of the physical layer to provide pricing that would allow third party competition.
Internet access is kind of interesting in that essentially the same product can be provided over different physical layers: The cable TV network, the wireline telephone network, satellites, and the cellular telephone networks. The biggest two providers of residential high speed services are the cable TV and wireline phone networks, which have regulated monopoly status due to the historical issues I described above.
Now there is an alternative that's popped up in a few places, such as Ashland, Oregon. In Ashland's case, the city itself built a high speed fiber network throughout the city. But the city itself isn't a content or access provider. Instead, third party companies can compete to offer Internet access through the fiber network. I'm not sure how the cable TV side works, but I know that cable TV comes over the fiber network as well. In that case, all the access providers compete fairly, since the owner of the physical layer doesn't compete with them. Potential issues with this is that the cost to build and maintain the network comes out of taxpayer dollars, so people who choose not to use the service are still paying for it. And in other cases where local government has attempted to build it's own network infrastructure, the existing providers have complained quite loudly.