The amount of energy needed to lift the Earth out of the solar system would have vaporized both objects. And to direct the Earth into the sun, the impact would have to be at a precise angle. I would have expected a major collision to result in a strongly elliptical orbit.
I get your point though, that we might expect a major collision to alter the orbit in some dramatic way. But astronomers do think the moon was formed by collision. I don't understand the mechanics either. I know our orbit is slightly elliptical, but not by much.
Here are my first 4 hits on a Google search for "formation of the moon"