in my experience, three things have been the reason - power supply, hard drive, or RAM. All which were replaced.
Yep.
Power supply is incredibly common, damage is incredibly common (liquid, impact or dust/debris), failure to maintain the equipment, shorts, etc. These are common reasons do for devices to fail. Hard drives have become better now that they're solid state, but HDD are still very vulnerable. I've never had RAM die, but if it does, that can corrupt BIOS, etc. Key thing among the above? Hardware, hardware, user error and hardware.
Luckily PC's are built to be modular and can have these components replaced, rather than buying a full new one. That works on phones too, but often it costs more to go that route than the device is worth because the industry is still so young.