Why don't motherboards power peripherals?

That's the way they used to be (not counting drive power cords), "way back when".  Then the processors started needing more power so they added the extra plug on the mb.  Both the main plug and the auxiliary plug have evolved/expanded over the years, I don't really know what's running on each at this point.

I believe the short answer is that there is only so much wattage you can pull through those skinny little mb wires until they overheat.  Extension cords are the same way - a thicker 12 or 14 gauge cord is rated for more amperage than the thinner 16 gauge cord.

I suppose they could run big fat wires around in there if they wanted to, and all the capacitors/connectors along the route would also have to be able to handle the extra load.  Probably makes more sense to just plug those porky graphics cards straight into the power supply though.

PCI allows 75w Max IIRC, modern mid to highend GFX cards use a hella lot more than 75w at load.

Ram/M2/usb/ect/ect all use far , far less wattage at full power.

What Dave said...;)

Besides, I wouldn't know what to do with a power supply that didn't look like an aggravated octopus   

whereas i've been wondering why ssd/hdd cables (power and sata) are so weirdly spaced. or at least i did every time i add a new drive or move them to a new machine, etc. absolutely no idea how to make the wires reach the ssd holder thing on the back of the mb plate as well as all the other drives. so i gave up and just stack them all in the drive cage rack thingy a couple of years back when i got the current one.

It is as DaveRI stated, there's too much resistance through the current copper traces to allow that high of a draw for power with it capping around 75w from PCIe at the moment.

That number has gone up a bit over the years as technology improves, but until a good cost effective alloy with lower resistance per inch comes out to replace copper we're probably stuck there for a little while. -- Scratch that if there's a major copper shortage which causes the technology to get catapulted forward

If you want to read a bit more on the subject

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3605

Now a different fun note, there are some video cards in the current generation that do not require an external 6 or 8 pin power connection, however they're typically on the low end and are constrained by the PCIe revision on your motherboard.  "Nvidia 1030/1050 cards for example" the big issue though is if you attempt to overclock them it'd draw more power which may make your system non bootable due to too much draw from a cold bootup.

I thought we would have computers powered by "the Ether" by now so I'm disappointed. My crazy expectations set me up for disappointment. Life is cruel because there is no guarantee to live long enough to see those things. Electricity seems so old fashioned. And generates heat. Time for something new I say.

Those liars of ours back when I was in school said 2010 would be future cool.

The gasoline engine would be a thing of the past, we'd get our own little rockets that would fly real fast.

I couldn't wait to start flying around, with a 4 hour erection that wouldn't go down..

I'd impress and undress every woman in town and I couldn't wait for that to get off the ground.

Now I'm sitting here there's a cop over there.

Me and him's supposed to be up in the air, him saying "Tim you're way to drunk to fly, I'll have to write you up for an FUI".

I'm stuck in an 89 Pontiac.

I'm supposed to be a flyin.. sex maniac...

So where's that contraption to strap on my back....

Where the F*#k is my Jetpack...

https://www.youtube.com/watch?v=r1SCu9yiBlo

^^  

Yeah, the bastards!  Lied about every fechen thing, didn't they!  Especially about the personal rockets and flying jet cars.

And me too!  I just wanted to get in my jet car and go visit George Jeston's missus while he was at work  Ooooh, she turned me on so... but a 2 hour erection would have done, given George's short working day and 'hi honey I'm home', being the coitus interruptus that would see me flying out the bedroom window without a parachute.

Mee too!  Providing I had a steady supply of cheap Viagra.  

In fact, I wouldn't have to be a sex maniac.  I'd settle for 2 or 3 times a night.