It's not "these use cases". It's the primary use case for the USB ports on this device.
The entire pitch for the Steam Machine is to use it as a gaming console. Needing adapters for the most common controllers would be nuts. There's plenty of C-to-A adapters for other use cases.
The Steam Machine is clearly not a high-end gaming console. I don't think most users will be using wired controllers. I could be wrong, of course.