The likelihood is that makers of flash packs all buy the leads from a common supplier with the plug ready moulded on. Given the volumes each are likely to use individually, it's unlikely that each company has made their own moulds and set up their own moulding line to produce a relatively small number of leads. It just wouldn't be economically viable. There is also a vast difference between buying in commercial quantities, usually on minimum order contract, and being able to get 'one-offs'.
It's not that I 'believe' it's not a common plug - I KNOW it's not a common plug. The only plug which fits the external power port of a Canon flashgun is this specific plug - it's an intentional design feature. No other will fit unless you start individual surgery / butchery on a similar plug. The nearest similar plug is the one which fits the Godox flashgun. Guess what? The same criteria apply to this variety also. This one only fits Godox flashes, only available ready moulded on the end of a cable.
The other end of the lead - in the case of Quantum, Godox, Bolt and Sky Eagle this terminates in a DIN 45326 plug - is intentionally compatible across a range of battery packs. To make this possible, they have to use a commonly available plug. The current favourite is the DIN 45326 which is actually an AUDIO plug and not even a power plug at all. Prior to this, there were a number of power packs available which used a 2-pin co-axial BNC bayonet-fix power plug but this didn't allow for the additional low voltage line needed by the latest types of flash - hence the need for a readily available plug with more than 2 pins. So we end up with a situation where the power pack has a circular DIN socket with 8 individual pin-sockets and a lead with plug using just 5 pins. The 5 pins will fit into a 5 pin socket or an 8 pin socket and no others. Using 8-pin sockets on the power pack socket gives a degree of 'future-proofing' and the flexibility to make changes without making all existing leads obsolete. It's not electronics, just ergonomics and marketing.
Voltage. Yes, we can measure the OUTPUT of the power pack. What is more difficult for us is measuring the input requirements of each type of flashgun. There is a degree of standardisation at present and a variety of power packs are able to power a variety of flash units, within specific limitations. There are also flash units which can, only after conversion of the power pack output, also use these same power packs. This is done with further voltage adjustment - usually in a little box part-way along the lead. If you research the Quantum leads for Metz flash units, you'll find a bewildering array of options. Lead "A" will only work with certain flashes, if you have a different flash then you might need Lead "B". This doesn't just apply to Metz flashes, it's just that this is the brand with which I'm the most familiar.
The final reason for the definite unavailability of the plug which fits the flash external power port is that, were they to be available readily, at some stage some idiot would couple up totally the wrong power source and harm themselves. "I wonder what happens if I couple this up to the mains supply" is very far from impossible to conceive.
A parallel example of the stupid things people do. A few years ago, a man using his power tool drilled through live cable. He died. His wife sued the power drill maker successfully because it didn't say on the drill "DO NOT DRILL THROUGH LIVE CABLE". Stupid and fatal thing to do which I would consider entirely his own fault. The Jury disagreed and awarded damages. Flashgun makers feel the need to avoid something similar happening to them, even though the voltages may not be lethal to a healthy person. What if that stupid person had a pacemaker fitted? Would the voltage be lethal then? Would you be prepared to take the risk? Damages, especially punitive damages, can be ruinous so the situation is avoided with 'unavailable' plugs.
All the preceding in addition to the need to protect commercial interests and hopefully steer buyers towards one make of product instead of another.
All-in-all, you're unbelievably unlikely to find one of the desired plugs on the open market. But if you do happen upon that one chance in millions, make sure you tell us all.
I just present it for your delectation, delight, satisfaction and as yet another gadget on which to spend your money.
