Because the Steam DevKit is defunct. The DevKit needs to be obtained from the Epic Games Launcher.
To avoid downloading the entire thing again, download the epic games launcher. Run it and start the install for the ARK Dev Kit, then wait until it starts downloading.
After it has actually started downloading the devkit, pause or stop the installation(I’m not sure which is required here) then exit the launcher and copy over the contents from where you installed the Steam version and start the launcher again and try again.
If it worked, it’ll probably want to verify and update to the latest version(it’ll be a big download).
-WM