There's actually something like that available - C-Band. However, if you're just picking and choosing the channels you want, they're generally a lot more expensive than their component cost in a package. The costs are so high, per channel, that the service offerings are pretty-much all gone, but here's a relative recent article (June 2012) that outlines the general case that prevailed when C-Band was more readily available:
Why Won't Comcast Just Sell Me the 16 Channels I want to Watch? | Free By 50
And that's over and above the basic account service fee (which IIRC was about $40 per month). So in the end, such an arrangement would only work out for people who truly only access four or five channels, max. Once you add a sixth or seventh channel, the package deals work out better.
So, with regard to cable television packages, as the article indicates: