Because the batteries aren't dead.
Leaving the remote control laying on its side for an extended period of time causes the electrons to pool on one side of the battery restricting the flow of electricity. Agitating the batteries by smacking the remote will loosen the electrons. Alternatively you can remove the potential difference by taking the battery out, which depolarizes the battery and removes the electron build-up.
Leaving the remote control laying on its side for an extended period of time causes the electrons to pool on one side of the battery restricting the flow of electricity. Agitating the batteries by smacking the remote will loosen the electrons. Alternatively you can remove the potential difference by taking the battery out, which depolarizes the battery and removes the electron build-up.