The answer to 'why is my shrub thin grass' is that there are several common causes, including poor soil conditions, lack of sunlight, and competition from weeds or other plants. This can lead to a sparse, thin appearance of the grass around the shrub.
One of the most common causes is that the shrub is competing with the grass for nutrients and water in the soil. Shrubs have larger, deeper root systems that can outcompete the grass for resources. This is especially true if the soil is poor quality or not well-draining. Another issue can be lack of sunlight - shrubs can create shade that prevents the grass from getting enough sunlight to thrive.