Valve uses its 30% cut extremely effectively by continuing to invest in the happiness and convenience of its customers and, just as importantly, its developers. For instance the profits of smaller developers dramatically increased after the new discovery/search features landed, enabling exactly the kind of people who would love your game to find it easily. Contrast that with every other store where it's basically impossible to find anything but the most mainstream top sellers.
Maybe Epic could afford to actually make the Epic Games Store a useful product if they took a higher cut of sales?