Its not a zero sum game here. 30% does seem like a lot to take, admittedly, but hey, they can give whatever %age they like, presumably the theory is that the marketplace will grow big enough to sustain that percentage. But you could just as easily host your own sale (lets say via gumroad) at a smaller percentage and sell via that if you think you can sell more directly. Epic are essentially acting like a portal at point (remember those casual games portals that used to sell indie games and take a cut?).
Still, if you think that percentage is too steep, you can always go sell elsewhere too. Which I guess is what a lot of the art content guys are doing.