They really need to fix that, I end up playing pickle with my opponets fleet, I chanse them from planet to planet over and over! Its anoyying!!!Would you prefer the AI fought to the death every single time? It's realistic, and sensible to withdraw from a fight it can't win.
It's not realistic to abandon a highly populated and developed world without so much as a shot fired just because the enemy has slightly more ships then you. The AI's planets are (presumably) closer, and thus it can produce and call in reinforcements faster which is what it should do instead of running.
As it is now, you just need a few more ships then the AI and you can tromp around it's empire stealing it's planets with minimal to no losses, then box it in it's homeworld and take it over with culture or just run it over with the huge fleet you have from taking the whole map over uncontested. It's a terrible strategy on the AI's part, and it makes for boring gameplay.