The thing is, there’s actually ways to enforce measures that mitigate spread in a haunted house. In the end, despite the people around you, and haunted house is a strictly individual experience. That’s fair less doable with parades and firework shows, which are communal.
Haunted house can pulse so that only one or two parties are in the house at a time, and remove surfaces that guests frequently touch. Scareactor positions can be replaced with automated effects and puppets to make sure guest and actor remain socially distant. These are things Universal can do that limit potential spread (things we know they’re doing), and they can proactively enforce with minimal negative impact to the event experience.
Outdoor events like fireworks and parades are a different beast. Disney would have to put social distancing marks, large enough for groups of three to six people, throughout the length of the parade routes and the entirety of Main Street so guests “know” where to stand, then have CMs walking through during the parade/show to enforce. That’s if they want to do it right. The scale of Meet and greets literally require engagement within 6 feet (otherwise, what’s the point), and there is no way to sanitize costumes between guests interactions.