There are all these iPhone commercials and they aren't showing anything special that Android can't do, so why doesn't Google show off their OS?
I'm not sure I get the "Apple is doing it so why isn't Google?" reasoning...
It would make more sense to me to assess if there would be any benefit to having Android itself advertised versus the current approach of having each device advertised. I tend to agree with jsh1120 that the masses don't make their purchasing decisions based on the phone's OS. Enthusiasts might and it might seem to make sense from the enthusiast (i.e. DroidForums.net user) perspective but enthusiasts make up a tiny fraction of a percentage of the entire market.
I'm trying to think of an analogous situation. The closest I can think of is if GM (or any auto maker) decided to market the different car platforms that are shared among its brands in addition to marketing the specific models. How many car buyers actually know the names of the platform that their car is based on aside from the car enthusiasts?
Even that example isn't all that great an analogy since the car platforms aren't handed off to external companies for them to create potentially different products. Hopefully someone can add a better analogy to the discussion.
Regardless, how many TV ads do you see for Google to begin with? Their recent Superbowl ad is the first I can think of.