The answer to 'why is my landscape lighting thin grass' is that the landscape lighting may be causing the grass to thin or die. Landscape lighting, especially when placed too close to the ground or not properly positioned, can produce excessive heat or shade that stresses the grass and causes it to thin out or die in those areas.
Some common causes of this issue include placing lighting fixtures too low, using bulbs that produce a lot of heat, or having the lighting positioned in a way that blocks sunlight from reaching the grass. This can lead to the grass becoming thin, patchy, or even completely bare in the affected areas.