They aren’t as efficient that way. Horizontal axis wind turbines most of the time have their whole “swept area” in unobstructed airflow. For vertical turbines the blades pass into the turbulent air from the other blades and the vertical post.
The swept area of a horizontal turbine is increases more for a given amount of additional blade length than a vertical turbine as well. Extending the radius of a circle creates more swept area than an equivalent increase of length or width of a rectangle.