People keep putting Steam on their desktop because Valve has spent decades cultivating customer's trust, meanwhile every other MBA has been burning it for short term gain. Partnering with Valve comes with access to things like APIs and tools, but you also receive a little bit of that trust by association.
Not only has EGS seemingly failed to build something with equivalent features, APIs, tooling, etc. but in doing so they've failed to show a reason to trust them. So if the lower cut isn't resulting lower prices, what's the value prop for customers? GOG has an angles: games nobody cared to make available for a long time and being DRM free. To me it looks like Epic took the MBA route and wanted to build a moat via console style exclusivity, but that's not trust. Free games aren't trust. Just being on the customer's machine isn't trust.
(Also, these letters aren't current. iirc Vale doesn't simply take a flat 30% cut in the current day)