

Ground is a very relative concept in electronics. You are correct about earth ground, but if the case is unplugged and you touch it, you are still grounding yourself to chassis ground. Chassis ground is the important one for this since the danger isn’t electrocution (assuming the PC is unplugged) but electrostatic discharge (ESD) that can damage components.
This is still a massive simplification of the concept of ground.
I have one like this but another who puts up with a lot. If I pet her or put a hand on her before I turn in bed, she’s fine with it. I think it’s because she then knows that I’m aware of her. I’ve also accidentally kicked her (not hard) a few times while tossing and turning and she didn’t care. But heaven forbid I sneeze or even inhale like I’m about to.