i don't understand why they don't do that...i bet it would be higher revenue for them...but the answer is no...
Ok - thanks for your replies. As 'Kelly11' said, Yes this will come up with more revenue for Google. They will not have to pay all the money people earn from adsense but that will be used for more advertisements. But may be this way google will be short of Adspaces due to too much marketing ads. Or they just want the two products to be fully separate than each other.