Uhm, their not reinventing the reel. The Clone Wars Designs were really old, so they had to rework them, to fit the newer techniques.
For the beginning of the Clone Wars, I can get behind that. They hadn't fought a war for a thousand years. The heaviest ship they'd have would be the Dreadnaughts, eighty years old at that time. They'd have to research more modern warships to compete with the Separatists. However, at the time of the Galactic Civil War, the Empire had not ceased military research and development since the Clone Wars. They already had Imperial I-class Star Destroyers, and according to some sources, had had them since the ending months of the Clone Wars. The Empire should, theoretically, have a full complement of classes from corvettes up to battleships.
Now I'm not trying to be confrontational. As I said before, this kind of thing is a long established trope. Also, it's not so much that they literally "reinvent the wheel", but more that they have to research things that they should already have.