Was curious about this so i had to look it up, Steam takes a 30% cut from game sales, same for GOG while the Epic Game store takes 12%.
Epic Games store set their commission low so they could fight their legal battle with Apple while trying to get more game developers and publishers to use or even switch to them.
Unfortunately, most people do not understand that for many game companies, the exposure to millions of gamers who are a couple of clicks away from buying their games at any moment is worth giving up 30% of their revenue.
If you were selling a product and a company with millions of customers who matched your product's demographic offered to help you for a cut of the sales, it would be hard to say no to that deal.
For 30%, they market your product to their millions of users (who buy products just like yours), they handle the purchase process, the transaction fees, the refund process, your licenses and all bandwidth costs. Even better, they provide you with a quick way to upsell your customers on expansions, DLCs, merchandise, digital items, etc. while providing an easy way to push updates to your customers.
I just wish they cared as much about their customers as they do their publishers.