Very simple. Star wars got it right (or at least, a bit more right than Star trek). Nebulae are very faint and without a big telescope and long exposure, you don't see anything. Compare with any real-world picture of a planet (or the moon). As long as you have a reasonably bright foreground object, the exposure is so short that not even stars are visible, let alone nebulae. The luminosities of spaceships/planets, stars and nebulae are huge factors apart, and even in observational astronomy, unless you want to see a nebula, you won't see it. And if there are nearby stars in the same picture, they will burn out the image (you'll see the diffraction spikes and bloom around them).

Very simple. Star wars got it right (or at least, a bit more right than Star Trek). Nebulae are very faint and without a big telescope and long exposure, you don't see anything. 

Compare this with any real-world picture of a planet (or the Moon). As long as you have a reasonably bright foreground object, the exposure is so short that not even stars are visible, let alone nebulae. 

The luminosities of spaceships/planets, stars and nebulae are huge factors apart, and even in observational astronomy, unless you want to see a nebula, you won't see it. And if there are nearby stars in the same picture, they will burn out the image (you'll see the diffraction spikes and bloom around them).