The water still moves through it, so it still comes clean, but you don't have the problem of, you know, it abrading, which - or in that case is felting.
So either there is some source for intact statues without abraded surfaces, somewhere that the archaeologists are missing, or as have CAT scans have shown, these are restored, shall we say.
But how often do we see along the bases of retreating cliffs rounded boulders, all thickly clothed by marine productions, showing how little they are abraded and how seldom they are rolled about!