Just going to sound really pedantic here, but RGB does not capture the entire colour space. In fact, it only captures about 35% of the colours the human eye can perceive.
https://www.oceanopticsbook.info/view/photometry-and-visibil...
https://www.oceanopticsbook.info/view/photometry-and-visibil...
However the entire color space of CIE RGB (1931) includes points where some of the RGB components are negative.
Because positive components are sometimes desirable (e.g. because one can make light filters whose outputs are those components), the alternative XYZ representation is derived by computation from the original CIE RGB, which had been obtained from experiments with human subjects.
Any RGB model can capture the entire color space, even sRGB. The limitation is not at capturing, but only at reproducing colors when using RGB emitters, because the emitters cannot reproduce components with negative values.
There are no RGB models that can capture the entire color space without having points where some components are negative. This is caused mostly by defects of the human color vision, e.g. by the fact that the red receptors are also sensitive to violet light, not only to red light, and by the fact that the selectivity curves of the photoreceptors do not have ideal shapes.
(At least, under the strong assumptions we make in color modeling using the CIE system of colorimetry; the basic keyword is "Grassmann's laws".)
Look at an object;
Observe colour X; then look at the wrong thing;
then look back and you might observe colour X' != X.