The answer to 'why is my garden thin grass' is that there are several common causes, including poor soil quality, lack of sunlight, drought, or pest/disease issues. Thin, patchy grass is often a symptom of underlying problems that need to be addressed.
Some of the most common causes of thin, sparse grass include: compacted soil that prevents water and nutrient absorption, insufficient sunlight due to shade from trees or buildings, drought stress from lack of water, or infestations of pests like grubs or diseases like fungus. Properly diagnosing the root cause is key to finding the right solution.