Why do many streams in the Namib Desert fail to reach the Atlantic Ocean?

In the Namib Desert, most streams fail to reach the Atlantic Ocean because they terminate in salt pans or mud flats (known as vlei) against or among the desert dunes.1

Because the region is almost entirely rainless, it features a poorly developed and fragmentary drainage pattern. While many of these rivers originate on the interior plateau and descend the western escarpment, they are intermittent watercourses that typically only flow for a few days every several years following heavy rainfall in the interior.123

Furthermore, a significant portion of the water from these major streams seeps into the sands of the streambeds, creating an underflow that is often tapped for water supplies. Only in rare flood years do rivers like the Kuiseb possess enough volume to reach the coast, while only the Cunene and Orange rivers flow permanently on the surface to the sea.123