The Emperor was never referred to as Palpatine in the original Return of the Jedi. However, throughout the aforementioned movies, he was never called by his real name.
Starting as an unassuming Senator from Naboo, Sheev Palpatine quietly sowed chaos that led to the collapse of the Galactic Republic and the fall of the Jedi Order, as revealed in the prequel trilogy. Born with a strong connection to the Force, the Emperor was secretly trained by Sith Master Darth Plagues, whom he eventually killed to recruit his own apprentice, Darth Maul. From there, Palpatine gradually amassed several followers who helped him achieve his goal of galactic dominance. That included Anakin Skywalker (Hayden Christensen), who was manipulated to turn against the Jedi and become the Dark Lord known as Darth Vader.
Despite appearing in all three eras of Star Wars films, there are still a lot of questions surrounding the origins of the Emperor. In fact, his first name, Sheev, wasn't revealed until 2014 via the James Luceno book Tarkin. That said, his last name was established even before the original Star Wars movie came out through the prologue novelization written in 1976 by Alan Dean Foster. So, why was the Emperor never referred to as Palpatine in the original trilogy? The reason could be rooted in how the films' overall arc was laid out. Palpatine was primarily presented as a master manipulator who operated behind the scenes. He never made public appearances and only accepted private visitors in his throne room. Given his limited interactions with other people, there was really no opportunity to call him Palpatine. High-ranking Empire officials like Darth Vader and Grand Moff Tarkin may have chosen to refer to him with his official title, while the rest might not even know his real name.
From a storytelling perspective, perhaps it was also a creative decision from Star Wars' story through the prequels where he'll be a main player.
Ultimately, not explicitly calling the Emperor Palpatine in the original Star Wars trilogy made his return in the prequels better. Fans of the space opera were already aware of Sheev and his role in the downfall of the Jedi Order, as well as, Anakin's transition to becoming Darth Vader given the clues laid out in the original movies. However, not explicitly associating the Emperor with Senator Palpatine made the villain's character progression more interesting in the broader scheme of things.
