No batteries last forever. Every rechargable battery dies after a period of time. Even the little AA ones you use with the Wiimotes or whatever. You will see that after a few years of constant use and recharge they need to be charged more often till they cant work anymore.
I know tha tin the case of the AA batteries this is because of the Acid that is in them starts to degrade over time and not hold the charge for long. I do not know how the laptop batteries work as per the interior. But i would guess it is along those lines.
Plus having the battery in if you are using a wall adapter will help kill the battery faster.