Earlier quoted context omitted.
This argument would be a great one in 2016. Now though, proton/wine works more or less for everything, and the storefront is a web based one anyway.
I'd hope this community of all places would understand that "just integrate X with Y" is never as simple as "just". It's still something a team needs to do, and the gain is minimal unless Epic is also going to try and make their own console-esque device. That's the incentive for Steam.
The same rationale exists for Epic, and they have spent an enormous amount of resources fighting Google and Apple over this.
I think it's an ideological decision rather than a technical one.