Well in the old EU, the Darth Plagueis novel basically laid out Palpatine’s whole life story, which didn’t leave much room for ambiguity. When they were originally designing the make up for the Emperor in ROTJ, the goal was to make him look not old, but “ancient.” I always liked the idea that we weren’t ever really sure where Palpatine came from or how long he’s been around. The “mask” concept allows for that ambiguity to remain.
It is a somewhat intellectual explanation, granted, but also I tend to think if you watched the saga in release order that the sudden reveal, rather than the deformation, would make more sense. At least I remember when I was a kid I always thought that what we saw in ROTJ was his real face, and when he showed up in the prequels he was just hiding it.