Why does the earth cast a strait shadow on the moon even though the earth is round?

The earth doesn't cast any shadow on the moon. The moon appears partially in shadow because sunlight only hits it from one direction at a time. It appears perfectly cut in two with a straight line when sunlight is hitting the moon from a 90 degree angle to the earth.

The only time the Earth casts a shadow on the Moon is during a lunar eclipse, and in those cases, the shadow, like the Earth, is round.