I was a city kid in dairy country of Wisconsin when I was in grade school.
And asked the same question. My country friends told me that was cows are stupid, lazy and creatures of habit, just like some city kids they knew.
When the cows was let out of the barn in the morning, the grazed as they sauntered out to the fields, so naturally they were all pointed "out". When they got to the fields, they were too lazy to mill around and didn't have enough curiosity to go investigate anything. Mid-afternoon, they'd start drifting back to the barn, chewing cud as they went, which meant they were all pointed the same way again-- in.
Now that I live in California, the Berkeley dairy farmers have told me a different tale. They say they've bred them to have their left legs shorter then their right ones. That way, they can comfortably graze on our hills -- as long as they graze counter-clockwise.
BTW -- Buffalo face into the wind and stand in bad weather. Texas longhorns put their tails to it and drift. Everybody knows that.
See? You didn't think there was a good explanation.
In fact, there's lots of them.
Fud