I made the exact same question for a farmer friend who came visit at 8am, when I was barelly waking up on my vacation. He explained that the cows will happily adjust their routine, but other factors should be considered.

A cow must be milked twice a day, ideally every 12 hours. So 8AM -> 8PM is late to finish the second leg and get all done for tomorrow. Milk is usually sold as fresh as possible, so 4 AM makes a lot of sense to get it ready when the market opens. At 4PM the sun will not be burning much and the farmer will be able to rest for the day, enjoy family and do whatever they want when this leg is done.