The explanation for the crop circles is: they are man made. Until there is proof for another explanation, the easiest, and therefore the most likely, explanation is the default one.

Again:

1) Why did they evolve in complexity in a matter of 30 years?

2) Why do they reflect the human culture present at the moment of their making?

3) Why do the "extraterrestrials" not place those designs where no doubt could be risen?

Please, I would like some answers from the crop circles believers.