Because we water the lawn.
Grasses that have evolved to tolerate drought have less chlorophyll than grasses in wetter environments. Chlorophyll (the pigment that absorbs red and blue light to make sugar from water and carbon dioxide) reflects green light, but in absorbing the other colors, it warms up.
If there isn’t enough water, the blades of grass close up the little holes (called stomata) where water evaporates to cool the leaves. If there is not enough water, the plant produces less of the chlorophyll molecule, and can survive the heat better.
In the summer, many grasses (especially in arid or semi-arid areas) lose their chlorophyll altogether. They turn the color of straw. In some the chlorophyll is lost because the sunlight breaks it down, and the plant does not replace it. In others, the plant actively removes the chlorophyll, and stores the valuable materials in another part of the plant. In some annual grasses, that part is the seeds.