There are two reasons for this:

1. The first is that turbines create a wake, an area where the wind slows down. The downwind turbine must be located sufficiently far downwind to where the wind has sped up again, and the energy generation does not suffer. The optimum spacing for rows of conventional turbines is seven to ten times the diameter of the rotor. Since that diameter can be as much as 400 feet, the spacing of the rows is up to 4,000 feet.
2. The second is that propeller-type turbines do not handle turbulent wind well. They must be placed far enough apart to escape the turbulence shedding from their blade tips.