A question about the movie Super 8 (spoiler alert)?

I liked Super 8. I thought it was good. But there's one thing that keeps me from thinking that it's -really- good. My question is, after the alien was freed, if it was able to collect all those materials on Earth, why didn't he just use them to build the spaceship in the first place? This really bugs me, and kinda is the difference, for me, between the movie being good and bad. Is this an actual plot hole, or is there something about the situation that I'm missing?