Games often write to Program Files, system-wide redistributables, or shared folders. VC++ runtimes, DirectX, .NET, PhysX installers all require admin.
Repack scripts are usually written to “just work” everywhere, so they have the stuff mentioned above built in. Or maybe fitgirl is just lazy and asks for admin even if it is not strictly needed.
There is definitely a risk of malware etc. when pirating, so you are not dumb to be wary.
Ideally you'd install the game in a VM first and run some virus/malware check on the whole VM afterwards.


