For a sense of scale, what is the background grass?

...and why is it so thin?

The bad grass on the left is about maybe 4", the one on the right is 6". Ignore the background grass, it's on the edge of where my neighbor did a quick reseeding late last fall (he'll do it property this fall) after having tree work done. This is also where the weed/bad grass is coming in.

My guess would be annual ryegrass. It grows very tall in the cooler weeks of Spring. Stiltgrass, on the other hand, grows tall in the hot Summer.