At the launch of Warhammer 40,000: Space Marine II, a lot of players were frustrated with the fact that Epic Games online services were automatically installed without their consent. However, in a future update, it has been revealed why there's crossplay with the PC platforms.
In a statement released to Eurogamer, Epic Games revealed that it is a requirement for multiplayer games on its platform to allow for crossplay with other PC platforms. "Developers are free to choose any solutions that meet this requirement, including Epic Online Services, which may require a secondary installation to enable the Social Overlay," said Epic Games.
Strangely, while this might be a requirement for all multiplayer games, there's one glaring example where it isn't true. Fortnite isn't available on another PC storefront, and while it is cross-platform, there's no crossplay on PC. Perhaps this could change if Fortnite came to Steam.