what, in your opinion, drives the start time of factory and manufacturing jobs to 5am?

I used to be an Operations Manager at a machine shop with 150 employees and a Program Manager at another places with 250 employees. Both had 5am starts. In my experience, the biggest factor was support for the billable staff. You have direct labor employees; these are the people who run machines and fabricate products. Then there are indirect employees that support the direct employees; like purchasing, planning/scheduling, management, customer service, quality, etc. Most manufacturers with a 5am start time are running multiple shifts of direct labor. The indirect employees usually don't start until 7-8am and overlap both shifts to have some support for both day and night shift.

Manufacturers that run one shift(like my current job) usually start later. We start at 7am but allow people to flex their start time for kids, etc.