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?