Because taking control of a Galaxy-wide Government grants you access to a pletora of resources which in turn permits you to give better equipment to your army.

In Star Wars our sympathies and loyalty as viewers switch between trilogies, yet diegetically the Old Republic, the Empire and the New Republic are a continuum, the same way after 1917 the bolscheviks didn’t sink any old russian Empire ship just becaused it served under the old regime, nor did the current Russian Federation get rid of old Soviet military equipment.

Which is also why Versailles wasn’t demolished after the French Revolution.

Or why Roman Cardinals are called the curia, which is a title that before the empowerment of the Church was held by the Roman Senate.

Etc. Etc. Etc.

OFC, it’s a personal opinion, but I just don’t buy that “evil shape” and “good shape” stuff. If anything, I would go with something that an old user from this forum always pointed out. The Empire is technological, massive, machine-like while the rebels are organic, small-scaled and put their confidence on individuals rather than on a system, whatever that is.

ST has the potential to show some of the conflicts you can face when you have to govern a vast territory with that philosophy.