Mixed feelings

Rating: 6/10 (1 July 2005)

I actually mostly liked this movie. The special effects were great. It had a nice blend of the original story and Orson Welles' radio play, and had a couple of nice twists of its own.
However, one of Spielberg's storyline variations left me puzzled and begs a question: Instead of landing in cylinders and erecting the machines (like in the original story, the radio play and the original movie), Spielberg has the machines already buried beneath us some time in our pre-civilized past and then has the aliens riding lighting bolts down to the machines to man  er  alien them.
My question is this: why would an alien civilization that wanted earth's resources bury machines and then wait for a civilization to conquer? Wouldn't it make more sense to simply conquer the earth to begin with and not have to muck around with all the burying and lightning riding? It's not like having cylinders land would have been technically difficult or would have taken the plot in a difficult direction. It certainly would have been a more plausible scenario than asking us to believe that an alien civilization wanted to conquer the earth, but not right now, so they're gonna bury a bunch of equipment just in case a civilization springs up in the mean time and then, hmmm, how do we get them down to the equipment? Eureka!! They can ride lightning bolts!! Did I miss the point here? Can someone explain this to me?