Rockstar has balance the money with an exclusive deal with Epic vs the massive install base on steam and the success they had with GTAV on steam. The amount of money that Epic would need to shell out would be considerable. Would epic be in the black if they tossed, lets say, $20 million to rockstar while rockstar also sells the game on their storefront?
i very seriously doubt it ... add one zero to that number and then maaaaaaybe