Potato harvesters are simple. They drive a horizontal blade through the ground at root level, and bring up everything above the blade onto a conveyor with slots. The dirt falls through the slots back onto the ground, and the potatoes are carried upward. Here's an operation that's digging 450 acres a day. Look how fast the trucks fill up with potatoes. No amount of hand labor could compete with that.[2]
[1] https://www.youtube.com/watch?v=i7gydrEpzAs [2] https://www.youtube.com/watch?v=0KL7OrHkDaY