Crop circles are created by a long distance force we humans haven't learned about. No credible evidence to the contrary has ever been presented.

But seriously, most artists couldn't even design the structures we have seen in crop circles.
The meaning, the depth, the compactness of information.
If the crop circles were human created, i would expect to see artwork of crop circles that were never made.

Further, crop circles, the real ones, are made by all the grass laying down. Not bent over. Not broken. Not trampled. The crop is still growing.

However, because we have free will, we are free to believe in crop circles or not.
Until we, as a planet, decide that it wasn't done by humans bending over crops, then it will always be in this gray area where you can believe or not.

BUT! If you want to keep your belief that these are man-created, then do not go looking into them. The information is far deeper and more complex then you would imagine. People who have come before you have said the same thing as you, until they started looking into it. Then it is like, OMG!

And what will you say when we create a universal transmitter and it looks just like that crop circle?

Unfortunately, we won't do that. First we have to accept that we aren't alone in the galaxy, then crop circles and how/who made them comes up, AND THEN we finally get to the transmitter. (but that's the limitations that free choice place on ya)