I'm surprised no one has mentioned the fact that when you bundle you are also reusing the same cabling to access the other portion. I would wager a decent chunk of the bill from a provider like Comcast goes into supporting the infrastructure, service calls, and truck rolls. If you have one service or three services the drops, splitter, poll agreements, etc are all the same. If a storm goes by and knocks out your tv it probably also knocked out your internet and phone as well.
With your same logic of using the same line then shouldn't all of their stand alone services be outrageously priced if you get just one?
Basic Tv $70
However, the reality of it is that you dont pay those prices for the other stand alone services as you would get them from someone else when possible. However, being that internet is very monopolistic, duopolistic if you are lucky, they run it the way the do.