They almost certainly move vastly higher volumes of milk than ice cream, making logistics a bigger concern.

It not that it's impossible to maintain the cold chain with other placements, it's that there is a cost (including that imposed by floor traffic) in doing so. Putting it someplace where a large volume walk-in cooler can be directly loaded from a truck and the display stocked from that without employee floor traffic across the store reduces the costs (labor, losses, and otherwise) of selling milk.