I really don't know why Fortnite guy keeps being thrown around as DLC when that troll post was debunked, but screw it, we need something new to talk about
He runs into the same problem Steve would: Building is a core part of their characters, and it severely disrupts the flow of combat in Smash. If they WERE in, their building mechanics are likely to get them banned.
For instance:
Build a walk off, bait your opponent to come near you, backthrow, profit
Build a cave of life
Build a wall to gimp recovery
You could compromise by making the structures super fragile or only last a few seconds, the former would work for wooden Fortnite builds, but the fact that they wouldn't last kind of defeats the purpose. Steve does have enough in his toolkit to make a moveset without building, but Fortnite guy's whole thing IS building, and again, if you remove building all together you remove the core identity of those two characters