Network effects make it difficult to move your contact list over to a new platform.
Also, Discord looks pretty is depressingly badly constructed software.
Plus having the largest game in the world (at the moment) helps build some inertia towards early adoption if they use that to funnel people into the system. I don't need to "rebuild my contacts" if Fornite suddenly requires me to add myself to the EGS, because that just sort of sorts itself out.
The Network effect can be used by Epic as well.
I've found historically with software that the better something works, the more likely it is to get screwed up. Netflix has made browsing a chore, Amazon Video introduced ads, Spotify is getting clunky as they "optimize" the UI, Gmail taking time to load in 2018 when it didn't in 2015, Android requiring root to install an app to prevent album art from replacing your lock-screen background and apps from making obnoxious color choices on notifications.
Ah well, maybe I'm just getting old and people want less robust, clunky software! /rant
https://www.gameinformer.com/2018/12/04/tim-sweeney-answers-...
https://arstechnica.com/gaming/2018/12/epic-undercuts-steam-...
> Sweeney told Ars that the company plans to "start small" and will "sometimes fund developers to release games exclusively through the store," Sweeney told Ars.