Sometimes the cost of an item is so high to manufacture and the demand for it is so low that the company cannot justify selling the item by itself because they would have to mark it up way too much to be cost effective to sell as a stand-alone product. The only way to make money on the product and still get folks to buy it is to package it at the manufacturer with something else. When that's the case, the reseller (BB, CC, 6th Ave, etc.) can't just rip open a package and sell you the cable because then they couldn't sell the rest of the package.
Either that or I'm WAY off 😛