Well you're not joking at all, that's very much true. You can't throw more cores at an antiquated game engine and expect them to do anything. Even modern bleeding-edge engines have a hard time doing anything useful with much more than 4 cores. We're only just starting to see a tangible benefit from having >4 threads in video games.
Actually FO4 is highly threaded, even so far as to use Hyperthreading.
Still, it doesn't prevent the random performance issues on PC.