Generally, these circular patterns are arranged in rectangular arrays of squares, like eggs packed in a carton. Arranging the irrigation systems this way gets you π/4, or 78.5%, of the wet area (and presumably the crop yield) you would get if you irrigated the whole square (which presumably requires more hardware and labor out of proportion to the change in yield). On the other hand, if you were to pack the circles in a hexagonal pattern (as often seen in mosaics), you would get an improvement in yield of 15% "for free".
One assumes most farmers are neither idiots nor disinterested in profit.
Why do American farmers pack circular fields in squares, not hexagons?
Religious beliefs require them to leave something for the gleaners.
Crop rotation: the corners just happen to be in a fallow period, and the pattern will shift next year.
Malnutrition caused by previous generations of poor farming has deprived them of basic geometrical reasoning ability.
Most farmers have small plots allocated in standard square lots and the overhead in cooperating with their neighbors to change the packing method is too high.
The Man pays farmers to farm badly.
Land is so cheap as to be basically free and farmers are farming as much as their labor and consumable inputs allow.
Sabotage: they are undermining the glorious corn-fed SUV revolution by underproducing. Damn kulaks.
Aliens: you're actually seeing negative crop circles.
You're just flying over the wrong farms. Most of them do pack efficiently, including using fractal technology to fill in the gaps with ever smaller irrigation circles.