As I understand it, the case of a PC is metal so that it can form an EMI enclosure around the electronic parts. My view is reinforced by there being conductive fingerstrips around the mating edges.
This is a home built PC without any form of case:-
I suspect that the black backboard with the four arms is scratch built too. It's probably plywood. This is 100s of Watts (motherboard + 2 high end GPUs) of totally exposed electronics running at ~4GHz spread out over at least a square foot. That's got to be an ariel somehow. There are many other examples if you Google "wall mounted PC". And there are of course all of the fashionable single board computers like Raspberry Pis running inside shoe boxes at 1.2GHz. Or glass PC cases.
Since EMI regulations get stricter all of the time, I thought that EMI is a problem. I read that it can be a major headache for equipment designers. Is it not really the problem the authorities make out? Or will this PC's owner never receive a mobile signal, be unable to watch clear TV or be sterilised within hours?
I've read EMI/RFI emissions and computer cases, Non-metal cases and emi standards/best practices for PC cases. The general tone of answers to these questions speaks to enclosing the PC in a shield of some sort, be it metal or conductive paint/ tape. Why bother?