Because I wish to reform it. As a member of the Dark Council I have been in a position to eliminate slavery, and to convince my peers that other reforms are necessary. The chaos that would prevail should one or both of the Republic and Empire collapse without one conquering the other first would be devastating. A galactic-sized power vacuum.
Of course, the Empire exists entirely so that one man can absorb all life in the galaxy in a quest for unlimited power, but that was, shall we say, a shock to learn.