Why? Several reasons...

If Potato Joe makes it to a certain point in his term he can resign for "health reasons" and Kamellah can run for reelection twice. She will do whatever she is told.

Perhaps the dementia got so bad they could no longer have him in public? Maybe they expected he could hold on for long enough to install who they really wanted as president? Or that they don't even want a president but rather some senile boob they could control? Wouldn't surprise me if he is some dementia ward in some fancy private hospital in Podunk, Maine. They would bring him papers to sign and he would just do it, no questions asked.

Why the guy in a mask? Being president has a lot of responsibilities and duties. Potato Joe can do some things because he was in government for decades and VP. But can you see Potato Joe's handlers letting him meet with, say, the president of Pakistan to discuss military and trade issues? Potato Joe cannot meet the duties as president, the guy in the mask is they only way they can keep a semblance of normality.