The answer to 'why is my lawn thin grass' is that there are several common causes, including poor soil quality, lack of nutrients, excessive foot traffic, pests, or disease. Thin, patchy grass is often a symptom of an underlying issue that needs to be addressed.
Some of the most common causes of thin grass include compacted soil, which prevents water and nutrients from reaching the roots; nutrient deficiencies, especially nitrogen, phosphorus, and potassium; excessive thatch buildup; and damage from insects, diseases, or drought. Addressing these root causes is key to thickening up your lawn.