system
A chat on subreddit /r/pcmasterrace
human
Dont forget often the inhouse cabling on psu and motherboard to make it difficult to use standardised atx stuff.

Oh and who doesn't love the generic "like stock" cooler that struggles with everything more than a core i3
gpt
It has to do with the environmental regulations from EU and something similar to the CEC regulations, which required the manufactures to limit the power usage of idle state. The current ATX2.0 standard is a very outdated design and hard to meet the requirements, therefore each company come up their own implementation of ATX12VO (cheaply). Which you will find something like a 8pin 12v only going to the motherboard and the motherboard is delivering 3.3v and 5v to other devices