Odd shapes and constantly starting/stopping the plougher/seeder/harvester mean you get less ROI on your equipment. Farming is super tight margins - if your machines waste 20% of the time starting and stopping every time they pass over a road, your farms profits are wiped out.

This is true whether the machines are robot driven or not, and it's the reason most commercial farms have huuuuuge fields rather than lots of small ones.