As Matt says, it's just not that concentrated. Moreover, it's not that lockstep. I spent a night at YearlyKos being bought drinks on the tab of a socialist working for Rupert Murdoch's Times. The Simpsons, with its obvious and often overt lefty bias, is happily aired on Murdoch's Fox. Why? Profit.
These guys didn't become magnates and multinationals through the consistent and determined application of ideology, they rose through an obsessive appetite for profit. That's why the media chases conflict, violence, fear, and sex -- the ratings winner is inevitably the network that looks most like Die Hard. Is that bad? Sure is, but it's not necessarily a problem of ideology. And it's not one that would be solved by less media aggregation, as smaller networks are even less able to pursue unprofitable strategies. PBS and C-SPAN, obviously, are free from such restraints, but they don't attract particularly massive audiences. Thankfully, the net's capability for low-cost content distribution is allowing a thousand niche players to bloom, and neither the capitalists nor the corporations can stop it. Unless, that is, they succeed in dismantling network neutrality.
