As far as I know, Steam's "exclusives" are Valve's own games.
No they aren't. Steam has tons and tons of exclusives that were not made by Valve, infinitely more than EGS has ever had.
Epic's exclusivity deals are restricting access to games which actually leads to worse options for customers. So I'm not at all a fan of Epic mostly for that reason.
Steam's many exclusive games are also very bad for consumers, and also worse for consumers than EGS' exclusives, because Steam has way more exclusive games.
So it's a double standard to see EGS as being bad for having exclusives, yet a turn a blind eye to and/or think it's totally fine for many games to be exclusive to Steam instead (although that is exactly what most Steam fans do).
I might be wrong but doesn't Steam has that many exclusives because the devs/publishers decided to release there only, while EGS has exclusives because they pay for them?
For example, Sekiro is Steam exclusive, but it has no Denuvo nor other DRM protection (besides Steam DRM), so couldn't Activision back in the day have released it here in GOG? And why isn't it here or on EGS 2 years later?
It seems to me that Steam was the only store to release on for many devs/publishers. GOG has existed for a long time, but the devs/publishers either don't care or only release years after. But again, I might be wrong because I'm not privy to the possible dealings done in the backstage.