So obviously I don't understand this market at all, can someone explain to me why google couldn't just copy ad-mob's business practices and features?
They did. Google's been running mobile ads (first in Japan, then worldwide) since 2006. But AdMob just plain does it better -- nicer interface and significantly higher CPMs for publishers. I'm sure they also wanted the talent. I've met one or two of the AdMob guys and they seem pretty sharp.
The talent angle of course makes sense, they are both pulling in top tier employees and removing top tier competition.