Actually, they can, because Cyberpunk 2077 can be pre-ordered on EGS with CDPR saying they want to make it available on as many storefronts as possible. This does mean that Cyberpunk 2077 is currently available almost everywhere; GOG, Steam, EGS for instance.
The problem is that the DARQ-developer, Unfold Games, also mentions that it's weird how such a hotly anticipated title can "simship" (simultaneous shipping; releasing at the same time) on EGS as well as other platforms just fine while Epic, after expressly going "Oh, you made a Steam store page. Come to us exclusively or don't come at all", deny this to other developers based on "careful curation" and "current plans".
I'm not buying their reasoning for it. It seems more like Epic just want to buy up small, good-looking projects with a planned release on Steam to expand their own store quick while preventing any small development studio from exposing sales-numbers. Sales-numbers would hold tremendous value if Epic ever agreed to a "simship" of a title like DARQ. With games like Cyberpunk 2077 they just accept the fact that:
- Either it happens anyway, or
- CDPR acts, like Epic expects, professionally to not reveal such numbers.
Epic are more than likely afraid to have a comparable set of sales-numbers held against the EGS while they still throw around Fortnite-money.