What’s your actual proposal here, make one “unit” of rope be 6", and allow it to auto-stack?
With thread, the typical use is to use tiny pieces of it for things (sewing), so it’s not that big a deal having 10 6" pieces vs 1 60" piece. it’s really an afterthought that allows it to be made into string and string into rope.
With rope, the typical use is to use the whole rope to do something, so the distinction between 10 3’ ropes and 1 30’ rope is HUGE, and that’s reflected by the fact that they’re distinct items. On the other hand, with work you can splice rope, hence the recipes. Actually splicing loses length, so the combining recipes should be lossy.