It's shitty code bound. Sometimes no matter how powerful your hardware is, software will perform poorly because it just doesn't scale. Writing complex software like game so that it can fully utilize current hardware AND actually run faster with better CPU/GPU can become very difficult once a certain complexity threshold is reached. It's easy enough to do for a small linear game even if it has exceptional graphics, but an open world sandbox game like ones that Bethesda makes is a completely different story.
That doesn't mean that it's impossible of course - Bethesda absolutely should have made a better job, but it's by no means an easy task.