And if you manage to beat both the Alderaanian fleet and the Corellian reinforcements, there will be more inbound from Corulag, or Anaxes, or any other pirate-owned planet the AI decides to throw at you.
Actually the AI can only throw Imperial/Rebel forces at you - IE the opposing faction.
Independent worlds are static and have defensive fleets only that can't move. The AI can however assess their strengths and plan accordingly, but in many campaigns - especially in the 18BBY era - your planets are the weakest in the galaxy and the AI prefers to attack you than the independent worlds....