Furthermore, Star Destroyers of the Imperial class are not able to fly in atmosphere like a Venator for example. Thats why they are build in space. They only have ION-engines build for use in space. This was also apparent in Episode 5. There the Star Destroyers could not launch an Orbital Bombardment, since the Rebel Base had a shield. So ground troops had to be sent.
The shield would’ve stopped any starfighters.