I remember reading an article many years ago that concluded that cheap labor is much more desirable than a fully automated system.
Small changes can easily be implemented quickly with people, but require a very expensive manufacturing, programming and testing process with humans. Just imagine, if they wanted to add onion rings inside the big Mac for example, they would have to get highly paid, mechanical engineers to retrofit the onion-ring placement array into the existing Big Mac assembly machines in all 12,000+ US locations. Or they could just send out a company-wide memo and the changes could go into effect the next morning.
Machines break, and they are very expensive to repair. Look at the airlines as an example of massive companies with lots of unusual specialized equipment. I worked for an airline, and while the planes were kept in great, safe conditions at all times, the ground equipment (some of which was 30+ years old) was always broken. We had 3 or 4 tugs (the little tractor cart things that pulled the baggage carts and other ground equipment) and often only had 1 or two operational at any given time. Also the belt-loader was always on the fritz and the air bottle (think of a jump-starter for jet engines) was unreliable at best. The point is that this stuff was unusual, specialized equipment that was difficult to replace, and repair. It requires well paid, highly trained personnel to service, and that’s just mechanical equipment. Picture highly sophisticated robotic parts and assembly lines with microchips, and many moving electrical components.
Lastly, I would argue that being a multi-billion dollar company, Mc Donald’s and other fast food companies have certainly already looked into this option and if it had been more profitable, I can guarantee they would be implementing it. Minimum wage is cheap, specialized robots and mechanical and electrical engineers are expensive.