The primary method of milking cows in the US has been by machine for decades...

And most farms in the US are highly automated: automated sprinklers, automated tractors, automated planters, automated harvesters (of crops that are amenable to machine harvesting).

It's simply that there are a lot of food crops that are trivial for humans to harvest by hand that would require very expensive machinery and technology to accomplish with a machine.

This. The supply of cheap human labor tends to make technological progress slower.

Did you have specific areas where there should be more automation? There is a lot of automation in farming.The tractors can drive themselves and planters continuously monitor and correct themselves. What is your definition of milking by hand? I suspect the percentage of cows milked by hand is extremely low. Even small family farms use milking machines.

You should do a web search for cow milking technology. Moral milk? DeLaval International, a large Swedish producer of dairy and other farming equipment, is installing 24 robotic voluntary – meaning the cows approach the stations on their own

I worked on a dairy farm for years. The jobs done by hand are done by hand for a reason.

The amount of hand milked product in the western world is miniscule. Probably there is a niche market but eco yuppies for hand milked pasture etc.