In Disney's Tower Of TerrorMade-for-TV Movie, why can't the ghosts just take the service elevator? Sure it's not "proper", but it works fine and you'd think they would've given up on propriety after the first twenty years or so. And they were apparently willing to take the stairs.
Since they took the stairs and it didn't work, they probably assumed that the service elevator wouldn't work either. The real question is how did they know that taking the main elevator would work.
They probably didn't know %100, but were clinging to their last hope.
This was addressed in the movie. The first thing the alive characters suggest is taking a different elevator. If they go up that elevator the party isn't there and they can't move on to their afterlife. Part of their curse was having to keep going to the party year after year in the elevator.
Because elevators are terrifying, that's why.
Given that this is based on the ride, why no connection to the Twilight Zone? Would it kill them to have a Rod Serling impersonator appear at the beginning and end to say something?
Because then they'd have to pay royalties.
This is the same exact reason why TOWER OF TERROR in Tokyo Disneysea does not have any references to the Twilight Zone due to Oriental Land Company not wanting to pay CBS to use the name. All elements of the Twilight Zone were replaced by a evil idol named Shiriki Utundu.