ATX POWER_OK signal not provided to molex connected peripherals such as HDD
The hard drive power connector ("Molex") predates the PC. It was in use already in 1976 and manufactured by Amp (not Molex). The connector was an established standard even before any of the PC, PC/XT, AT or ATX power supplies became into existence.

Hard drives and such are connected to the motherboard via a control bus (IDE, SATA,...) which provides them the necessary resets and commands. It wasn't really necessary to force any changes to the established power connector, because the hard drive is essentially slow to start up and doesn't need to do anything before it receives a reset from the motherboard, although they can spin up the disk if they so choose. And in any case, hard drives were already manufactured without the need for any "power good" signaling from the power supply. 

Peripherals can be fitted with their own voltage supervisory circuits, if needed. Also the power supply could be designed to not to provide power to the Molex connector before the +5 volt and +12 volt supplies become stable.