Why do you have to pay for cable channels you don't want?

That's the way cable companies make money. They simply charge a fixed amount for a fixed amount of channels whether you watch them or not. Obviously you can choose packages, sometimes called basic, limited, premium. Then there's digital packages that offer more choices.

If you're lucky enough to live in the right area, Comcast offers packages, like "News", "Kids/Family", "Movies" that offer only those channels that fit that criteria, for instance, if you just wanted the "Kids" package, all you'd be able to watch would be Nickelodeon, Cartoon Network, Disney & other kid related channels.