"They’re not able to focus on the high-intensity work" That is true. most 30 and over people with work experience know devoting 9-12 hours a day to a company is not worth it. While, a young engineer right out of college doesn't know any better. Burn them out after 2-3 years. rinse and repeat.

The main problem is employees are "at-will." Meaning they can be fired at anytime without reason. companies have no incentive to not overwork employees. Laws regarding "at-will" need to be changed