It's not like the apple app store where there is no other option, or even like the android play store where enabling non-play store applications requires the user to find a hidden setting and tap through deliberately scary popups, steam has no control over where you install software from on windows or even on the steam deck which runs a full desktop linux
Can devs implement denuvo selling a straight .exe like that? I thought the appeal of selling on Steam or Epic was all the antipiracy stuff that only like three people on earth have cracked (fitgirl, empress, and the other one)
Well, first of all, if devs want to implement denuvo they should be shot.
But from a purely technical perspective, denuvo is not affiliated with steam or epic, which have their own DRM measures. Steam is fundamentally a DRM scheme, it's just a DRM scheme which tries (and largely succeeds) to ingratiate itself to users by doing things like handling multiplayer and achievements.
Also, fitgirl doesn't do cracks, fitgirl does repacks, putting the game and the cracks that others have done all together into one simple easy to use package.