Why aren't stars visible when earth is filmed from space?
Because of the reflecting light.
As you probably know, we can observe other planets from Earth because the light of the Sun reflects from the planets and back to us.
When a picture is taken by a probe that we've sent out into space, the reflection of the light of the Sun hits the Earth and back into the camera. The light from the Earth is stronger than the light of the stars in the background, therefore we can't see them.
Imagine that someone is standing infront of you with the sunlight behind him/her, you can't see that person, it will only be a black silhouette. This is because the light from the Sun is stronger than the light hitting this person and reflecting back to you.