Because if it breaks, there are no spare parts, the people that maintained the system are long gone and the software that it ran probably does not run on a modern computer/OS.
The oldest box I've personally seen runs Windows 3.1 it is a specialized measurement instrument (Microwave radar) with a hardware card that runs in an ISA slot.
There are also a fair number of RS/6000 Unix boxes which are getting pretty long in the teeth.
For spare parts I've heard sometimes engineers buy things like Scsi hard disks off of ebay.
The oldest box I've personally seen runs Windows 3.1 it is a specialized measurement instrument (Microwave radar) with a hardware card that runs in an ISA slot.
There are also a fair number of RS/6000 Unix boxes which are getting pretty long in the teeth.
For spare parts I've heard sometimes engineers buy things like Scsi hard disks off of ebay.