Actually, you have to ask George Lucas for that. He did it in ROTJ when Vader throw him down, he was still shooting lightning. Even in ROTS, when Mace Windu was fighting, blocking and pantul his force lightning, he did not stop until it disfigured him.

Palpatine is too confident of the Dark side. So that is why whenever he uses lightning, he won't stop because he thought he is Ip Man. There's another explanation is if he stops, the Jedi who is attacking him can easily strike him down with their lightsaber. The lightning acts like a shield to him.