Taz, they didn't steal the designs because they couldn't come up with it, it's because they needed to begin on something fundamental.
Imagine what would have happened if the American engineers at the time of the WW2 when they were facing superior (At the time) Japanese fighters said:
lets spend more crucial time creating new and original planes that just MIGHT work against the japanese!
thankfully, that wasn't the case, and I'm not speaking japanese.