Why are most businesses open 8-5, when most people work 8-5? ?

They only have one shift of workers per day,  not 2.

You just answered your own question.  

Since the majority of businesses are open from 8 to 5 - that must mean that a large majority of the work force also work those hours because the have jobs at the kind of businesses that are open from 8 to 5.

Because people work 8-5 and if the business was open outside 8-5 the employees wouldn't be there. I could make some good money on weekends but then my employees would miss all the weekend activities with friends and family.