Maybe, just maybe, there's a difference between planning and installing something new in a location where it was never meant to be in the middle of a still-operating attraction and re-assembling an event that mostly uses live actors year after year with all of the necessary infrastructure already in place.
That would be true if the scenic and design did not include effects, performers, technitions, stage management, liscencing, marketing, casting and technological effects that are in the dozens every year including stage shows, custom puppetry, pneumatics, animations, audio visual and new venue locaitons construction and strike that go before and after the even'ts operating time that only gives less than ten months between opening and closing. dates The event is just not just reopened with the same or new actors in the same attractions every year as you make it sound.
There are years when entirely new haunted attraction venues are opened as the event has grown. It is sitll all more effort than a single figure, and the attraction did have downtime for other reasons recently that would and should have given them days to install this without the attraction operating.
If you want other exmaples there are more.
Transformers was built in the middle of Universal Studios Florida, building demolished and new attraction built in under years.
How was Tower of Terror so changed in the early 2000s?
Olivander's was redesigned as all of Hogsmeade operated around it.
Your argument is particularly awful as Haunted Mansion Holidays was able to put things where they were never meant to be thorught an entire ride on a grand scale over 20 years ago.
Retrofitting is one thing. But that never meant to be logic makes no sense. Nothing built in a theme park was ever meant to be. They already are placing it where it was decided it would go. Those curtains have also been up for a long time. It is just not a priority compared to other things. People have to admit that they would rather run the attraction long hours without much push to get this thing done by the time its movie came and flopped or Halloween season.