Why can't epic do normal things with all thier fortnite money, like buying expensive cars and throwing absurdly lavish parties or something?
Because, like Cobra, they're determined to rule the world.
In all seriousness, Epic is making a long-term play to try to gain market dominance through legal shenanigans. What they're going for is the courts outright saying that simply declining to do business with Epic is illegal collusion with Valve's supposed monopoly, i.e. that developers are now legally mandated to do business with Epic on whatever terms Epic cares to set.
Once that legal precedent is set, Epic can flip its business model so that the reduced margin for the Epic Games Store comes out of the developer's pocket (i.e., the lower commission is balanced by forcing the developer to lower their prices accordingly), which means the profit gained by undercutting Steam's prices will go primarily to Epic.