The ARK Dev Kit and the game itself are integrated with Steam Workshop. When you cook and upload your mod, the dev kit creates a new Workshop page and that's where it uploads the contents of the mod to. The mod is not really packaged in a generic form that you could easily distribute outside of the Workshop, and the way mods are loaded, they're designed to work only as a Workshop mod.

Also, SteamCMD is what is used to download both the server files and to update mods and so having the mods somewhere else would be useless as SteamCMD can only download mods from Steam by their Workshop mod ID. The clients that join are then required to download the same version of that mod ID from Steam.

You could manually transfer those files and such for SP perhaps, but the game is still only going to load mods by their Workshop mod ID and because it's the server who decides what mods to load, the mod you want to distribute that way still needs to have a valid mod ID on the workshop and will be automatically updated from the workshop repository when you try to launch the server/client anyway (negating the value of having, say, a Nexus version of the mod).

As for mods being outdated, that is often an issue because whenever a new expansion drops the Dev Kit is not directly updated alongside it. It may be a month or months before we see the Dev Kit updated for the new Genesis content, for example. Having mods on the Nexus wouldn't change that. Another reason they may be outdated is as simple as the author having given up on the mod, etc.Only the author has access to the source files for the mod, so only they can update/cook/upload a new updated version of their mod. If they stop working on it, then there's nothing that can be done about it; it will remain outdated indefinitely.