The answer to 'why is my garden thin grass' is that there are several common causes, including poor soil quality, lack of nutrients, too much shade, or pest/disease issues. Thin, patchy grass is often a sign of an underlying problem that needs to be addressed.
Some of the most frequent causes of thin grass include compacted soil, nutrient deficiencies, too much thatch buildup, and problems with the grass type not being well-suited for your climate or growing conditions. Proper lawn care, including aerating, fertilizing, and overseeding, can often help thicken up a thin lawn.