Why can't you see stars in space?

If you mean from here on Earth, it is usually due to pollution or "light pollution" (excess light) which causes eye pupils and camera irises to adjust for brightness, thus keeping the dim starlight from being seen.

Astronauts in space can see stars, so long as there is no bright light to force their pupils to contract, thus letting in less light. Science Fiction shows make people think that once you are in space, stars are bright enough to be seen through a window in a well-lit room. This simply is not true. The astronauts on the lunar surface didn't see stars as it was daylight and the Sun on the lunar surface was simply too bright for their eyes or cameras. In fact, their helmets had "sunglasses-like" darkening, as the Moon gets more sunlight than any desert on Earth.