Looks great on paper but the real world chooses differently, look at some farm crops, long straight swaths, easy to manage.
A human would spend so much time trying to re evaluate the lawn as he goes it would lead to wasted time and inefficiency. Also machines may not be able to physically follow the "ideal" line.
All that turning would:
slow down the machine
introduce a lot of overcut of areas that have already been mowed. ( not to mention that a machine may not be able to make the turns required to be 100%
Straight lines maximize the amount of grass cut over time and reduces complexity so the operator can drink beer.