It's totally bizarre that our cultures have gotten more separate in terms of the media we consume.

Is it? Marketing and advertising seeks to sell as much product as it can, but not all products are inherently appealing to all people. Furthermore, marketing and advertising seeks to develop groups in such a way that a lot of product can be sold. If the TV viewing public are all watching one show, then that's a problem for other networks, who are also in the business of providing entertainment. It's also a problem for that network's sponsors, who run commercials they've crafted to appeal to the demographic watching the show at that time slot.

However, it's easier to appeal to a niche market than it is to the public at large. Thus, we have a million channels and a million different things being sold on those channels.

Of course, this is a gross oversimplification, but I definitely think that advertising and marketing has played a role in how people consume media and how media demographics are conceived of.