I was wondering why so many games have recently had the "Epic Games Store Exclusive" label on them-- including "Shenmue III", which I'm part-way through, and "MechWarrior 5: Mercenaries", which I'll start tomorrow. So I took a look at their "about" page, and the math is pretty clear.

If you use Unreal Engine for your game and publish on Steam, you lose thirty percent to Valve, and another five percent to license the engine. Publish on their store, and you only pay twelve percent, while you get the engine for free. Their FAQ also says that they offer some form of financial support both for development and marketing, if you exclusively publish with them.

Not counting tax confiscation and the usual fixed and variable costs of doing business, if you sell one hundred thousand copies of a game for fifty dollars a piece, publishing on Epic's store instead of Steam nets you an additional 1.15 million USD in profit. And that's not even counting their marketing muscle.