In the case of the Haunted Mansion, even if you don't go through the front door, you're still entering the physical building. Sure, it isn't hard to figure out that the attraction doesn't take place inside that specific building, but that's not the point.
In the case of the Falcon, if you don't enter the Falcon that's parked out front- but instead enter into a different door, that'd be like entering the Haunted Mansion through this door:
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Sure, it'd still be an excellent attraction- but entering a building that looks the part is essential to setting the story.
And if you don't enter the giant Falcon Disney parked out front, but instead take a different entrance inside a showbuilding, Disney's gonna have a hell of a time creating and explanation that makes sense as to why the Falcon you just saw parked out front is now in a different setting.