From my observing in the Namibia space an understanding of how this and thus other sand deserts have come to be:
In nature moving is what sorts, the big from the small and this is exactly what flowing wter masses do.
The detritus moved by flowing rivers, more markedly observable in perrennials, is sorted by the flowing water masses according to their grain size and depositted in that configuration when the flowing waters flow out from the spaces between the grains.
The large sand deserts are such detritus deposited by flowing water masses: “Washed clean” of swmaller organic material, one big collection of homogenous grain size sand particles.
The spacing in this homogenous grain size conglomerate cannot retain water, it simply drains down into the earth, away. No plants grow and no organic matters is formed to fill those gaps. therefore the desert stays deserted.
Each and every year this phenomenon repeats in the dry riverbeds of Namibia when rainfall inundates the perennials, the water flowing quickly and sorting the detritus, leaving behind swathes of homogenous grain size collections, and where these are sand, typically in the stream middle, no water is kept, no plants grow and that area always stays dry.
The size of the mountainous deposits of sand in the large deserts is an indication of the amount of water that must have flown here to achieve this scale of deposition.
Should follow the notion that these sands have been deposited over aeons by winds, i.e., are aeolian, look at this:
the deposits of sands in the Namib, when viewed from 9000m above ground level are identical to those found, at smaller scale, in the perennials after each new run-off. Also where they lie indicate water as the causative, not wind. wind deposits sands behind and on hindrances, flowing water deposits sands between hindrances – there is always a small gap between water borne and hindrance.
Short and sweet. 😉