External power supplies and DC to DC converters for Computers and notebooks
While the CPU and GPU works on 12V

That is not true, in a desktop PC you will see one or more 12 V connections to CPU and GPU. However, these go straight into DCDC converters converting the voltage to one or more low voltages. Often 3.3 or 2.7 V for driving external (to the CPU/GPU) chips and 1.2 - 1.9 V for feeding the CPU/GPU.

GPU and CPU chips cannot handle 12 V. These modern, high density CMOS chips need less than around 3.3 V and 1.9 V for their supplies.

...and the SSD works on 5V

Likewise the chips in an SSD often work at 3.3 V or lower. Only magnetic based harddrives need 5 V. The 5 V on the connector is mainly for backwards compatibility reasons.

So in a desktop PC as well as in a laptop, DCDC converters are used to make the low voltages needed by the chips. There are several reasons for this:


CPUs and GPUs can run on a lower voltage when not much performance is needed so power is saved. By having the DCDC converter close to the CPU/GPU the CPU/GPU can more easily control it, also changing the voltage will not influence other chips. The DCDC is only used for that CPU/GPU.
Having the DCDC converter close to the GPU/CPU helps to get a more stable supply as it avoids long wires. Current loops remain small which is  needed to reduce EMI emissions.
And the big one: efficiency, if a CPU/GPU needs 48 W then at 12 V that is 4 A. But at the 1.9 V which the CPU/GPU actually runs that is 25 A. That would require very thick wires to limit the voltage drop.


That last reason is also the one why notebooks use 19 V instead of 12 V. OK, it is not that much difference but it helps. At 19 V the current will be smaller meaning a slightly thinner cable can be used.

Another reason is the laptop's battery. These are often 11 V but some are 14 V which would be a hassle to charge from 12 V. The 12 V would have to be increased a bit first. With 19 V that's not needed.

GPU on an external adapter
In theory, yes that would work.

In practice: it will be a challenge to get 12 V at the required current without too much voltage drop at the graphics card.

Using 19 V: not a good idea. Graphics cards are not designed for using 19 V. You might damage it.

ATX using only one 12 V line

No you cannot, these supplies are not designed for that. The load has to be shared between both lines. This also has to do with reducing the voltage drop by using multiple connectors and cables.