The way one designs hardware in is to optimize for the most common usage scenario with enough capacity to account for the peak use scenario (and with some safety margin on top).

(In the case of silent power sources they would also include lower power leakage in the common usage scenario so as to reduce the need for fans, plus in the actual physical circuit design would also include things like airflow and having space for a large slower fan since those are more silent)

However specifically for power sources, if you want to handle more power you have to for example use larger capacitors and switching MOSFETs so that it can handle more current, and those have more leakage hence more baseline losses. Mind you, using more expensive components one can get higher power stuff with less leakage, but that's not going to happen outside specialist power supplies which are specifically designed for high-peak use AND low baseline power consumption, and I'm not even sure if there's a genuine use case for such a design that justifies paying the extra cost for high-power low-leakage components.

In summary, whilst theoretically one can design a high-power low-leakage power source, it's going to cost a lot more because you need better components, and that's not going to be a generic desktop PC power source.

That said, I since silent PC power sources are designed to produce less heat, which means have less leakage (as power leakage is literally the power turning to heat), even if the with the design having been targetted for the most common usage scenario of that power source (which is not going to be 15W) that would still probably mean better components hence lower baseline leakage, hence they should waste less power if that desktop is repurposed as a NAS. Still won't beat a dedicated ARM SBC (not even close), but it might end up cheap enough to be worth it if you already have that PC with a silent power source.