Because R-R-R-Ruffles have R-R-R-R-Ridges.
My guess is it makes the fit sturdier when used with concrete, makes the bars less apt to loosen with age and rotate. Also, the flat horizontal bars are likely fitted to the ribbed shape increasing stability.
Keep them from turning in the cross bars?
Maybe they didn’t weld all joints to leave slop in case of foundation settling.