Force users, in combat against other Force users, innately defend against other Force abilities, forcing them to defer to the use of physical weapons instead. The Yoda vs. Dooku duel at the end of AotC makes this very clear.
At the same time, if there is a sufficiently large difference in power between two opponents, then lightsabers are in fact superfluous, since the stronger Force user can simply crush their opponent using their abilities directly. SWTOR contains numerous examples of this, with beings like the Dread Masters just killing people with their brains and Valkorion mind controlling even Jedi Masters at a whim.
During the confrontation between Luke and the Palpatine in RotJ the difference is indeed high enough that Palpatine can freely blast Luke with his powers (and, hypothetically, could have simply shunted Luke's lightsaber aside if Vader hadn't blocked it). By contrast, during the duel between Palpatine and Mace Windu in RotS, the difference is much less and Mace successfully blocks Palpatine's lightning.