Epic can take "some money" by selling third party indie and AAA games... or take "all the money" when people spend in Fortnite. It's a conflict of interest: Epic don't want a good store that do the job for other companies, Epic want advertisement for their single product. They give free games with the same logic you get free merchandise to gather people around place that cost money... they don't give the customer free stuff to make them happy, they don't give "free money" to publisher/developers because wants them happy (well, aside for the purpose to have happy business).
They want as much as people possible, regardless of their role as customer or publisher, to bring their business in their pocket.