Epic pays you, ever though what are they actually buying from you as Dev/publisher. Epic store don’t make the mony Epic is spending… that’s because sales on the Epic store aren’t even their goal. EGS is basically a huge advertising for Fortnite, games published over there are accessory to the ever present/default Fortnite’s events/promotions. On steam page for GTAV you see ads for Saints Rows, on Saints Row’s Steam page you see ads for GTAV. On EGS Fortnite is always omnipresent: the goal over there is not gamers buy as much games possible, but rather yell “hey! Free stuff? We have free things… Also Fortnite!”. It’s a black hole where wallets are swallowed by Fortnite.
“EGS vs. Steam-monopoly” is a totally faked presumption. What we see is that quite the opposite is happening. Exclusivity damages more all small and big competition around Steam: itch.io, GoG… but also bigger stores from Ubisoft and EA (which saw fair amount of investment in their own PC store in the pre-EGS era and now are mostly forgotten). Basically EGS is digging a more monopolistic trajectory for Steam. Indie are wondering “why should I publish on itch/GoG if Epic pays me?”