As much as I wish the same, (ie. The Vista style oobe was great for this, and customizable, I think it can be added back through OPK, but not sure), i think it was either not a viable solution or,,,,,
Being Accused of Anti-Competitive tactics.
They would no doubt come under heavy fire for such a move.
In my opinion, 7 should have come with Microsoft Security Essentials as part of the native install, and the first act of logging in for the first time is have it update through Win Updates before the system does or connects to anything else, for an even stronger defense against viruses straight out of the gate.
But they would be dragged through the courts for that as well.
Apple could get away with it no problem. But not MS, oh no.