We've even heard that about the Pyramids. Seems some people sell the human mind and ingenuity very short.
Cripes! We've been to the moon and space, to the depths of the ocean, mapped the human genome, and a host of other accomplishments. why should making cicle designs in crops be so difficult?
I never said or even suggested that crop circles couldn't be manmade. Can't speak for anyone else. The accomplishments you mention above weren't done by a handful of people working without support in secret. That's what seems unlikely.
What amazes me is how provoked so many people seem to be by the mere asking of logical questions. Some (not you) seem to want to shut down any critical inquiry from the get go. That's a superstitious mindset in my POV.