The answer to 'why is my landscape lighting thin grass' is that the landscape lighting fixtures may be causing damage to the grass. Excessive heat or light from the fixtures can stress and thin out the grass, leading to bare or patchy areas. This is a common problem that homeowners face with landscape lighting installations.
The solution is to evaluate the placement and type of lighting fixtures, and make adjustments as needed. You may need to move the fixtures further away from the grass, use fixtures with lower wattage bulbs, or install the fixtures in a way that reduces direct light and heat on the grass. Proper maintenance, like aerating and overseeding the lawn, can also help the grass recover.