Taris in particular hurts because that one is more of a SYMBOLIC victory. It doesn't hold some great secret to the force, an important resource, or even act as a staging ground for further expansion! The people in charge just don't want to see it rebuilt because Malak destroyed it, and they think it's a good symbol of Sith strength!
It gets worse, because kicking the Republic off Taris gets Saresh, who is apparently much more competent than Janarus, back on Coruscant in the Chancellor seat. So in the long term, Taris was a win, but the fallout actually hurt
And no, Saresh is not in the Star Cabal. But that also makes it worse for the Empire. The Cabal wants both sides destroyed...Saresh on the other hand only has one front to fight.