The answer to 'why is my fence thin grass' is that the fence is likely blocking sunlight and airflow, preventing the grass from growing properly. The shade and reduced air circulation created by the fence can cause the grass to become thin and sparse, especially along the fence line.
Other common causes include soil compaction from foot traffic, poor soil quality, or damage from pets or other animals. To fix the issue, you may need to adjust the fence, improve the soil, or overseed the affected area. If the problem persists, it's best to consult a professional landscaper who can assess the specific conditions and provide tailored recommendations.