[cucuuc]

"Is the wind not as strong on land?"

Yup !

The turbines are anchored to the sea bed. The wind has a boundary condition against the ground. If the ground is the sea, then the sea current wrt to the sea bed adds to the wind speed wrt to the sea bed.

Therefore, the turbines have zero wind speed only if the wind is blowing exactly against the sea current. Which is hard and rare.

Because the land boundary condition is different, the further away offshore you are, the better the effect is. Also, you might as well pick spots with known stable sea currents.