As a resident of Delaware, getting a second (we still haven't resolved if that exists) or third shift going isn't as easy as it seems. Just finding employees in Delaware is a very hard problem, as we have one of the lowest un-employment rates in the country. In my area, jobs go begging. To fill even part-time summer jobs , they pay well over minimum wage (usually $7+/hr), and these jobs are vacant. Over 2000 foreign students come from Europe to fill some of them. The State Department of Transportation has over an 11+% vacancy rate! You are lucky to get even 1 bid on a home construction project. And the list goes on.
So staffing extra shifts is a problem. If you found staff, training is not a short-term process. Then there are the extra taxes I'd have to pay if they closed some production, and went on unemployment.
I am sure the wizards in some GM financial planning headquarters have this all modeled out. They can plug the numbers into a computer program, and find out when it makes sense to add production staff. If they over-hired, the price of the car would surely go up..