Because it's pointless? I don't even think iPods are selling as much as they used to. They've been relegated to dusty corners of the Apple Stores as far as I can see. The main selling point of iPods now is for people who want a portable Apple device, but don't want to buy a current gen iPhone, considering those are twice the price of a same capacity iPod. You can keep a basic cellphone and use an iPod together, and you have a lot of similar functionality with an iPhone, at the problem of needing an additional charger and two devices to carry around.
I've yet to see anyone who has both current gen iPhone and iPod. Most of those I know who have an iPod and iPhone either have different gen devices (I know someone who has an iPhone 3GS still, but uses the current gen iPod because he didn't want to spend so much on a new phone), or someone who has an older gen iPod and decided to buy an iPhone instead of a new iPod.
In an Android scenario, a music player device isn't ideal. The plethora of cheaper Android devices makes them irrelevant. Why will you buy an Android music player (Sony still has Android Walkman lineup I believe), when you can just use your old phone? Or a second hand phone? Or a cheaper device from ZTE or Huawei? You can get a quad core ZTE device running JB for like $150, way cheaper than how Samsung or HTC or Sony would price their offerings, considering they also have cheaper devices to sell, like the Galaxy Fame. Take a look at say, the cheaper end tablets. A Galaxy Tab 3 WiFi sells for just $30 less than a Tab 3 with 3G. If we take this to a player scenario to a Samsung phone, for example a Galaxy S3 Mini, which currently sells for around $250 unsubsidized. Making a player version with the same specs would be, $220. So why bother? A thinking customer would just buy the unsubsidized S3Mini and he can use it as a player by just not putting a SIM card in it. Plus he can use it as a backup phone should his primary phone's battery get drained.