Tony, you have never come across HP / Compaq / Dell or IBM desktop kit...
Unique fit / bespoke mobos, power supplies & connectors, drive brackets, case wiring connectors etc. Even unique floppy drives in many Compaq models!
OK, I'm talking here about business rather than home ranges, but even home Dell PCs have (or had) non-standard PSUs, motherboards and mounting kits to my certain knowledge.
SD