So they tried for a moat and failed so that is why there is no evidence of one.

OK, but you could apply that argument to the segmented ditches of causewayed camps too. Difficult to flood the ditches on top of Windmill Hill, but they would look grand if flooded and reflecting moonlight. Nice to speculate, but we can only strive for answers by trying to interpret evidence.

Personally, I can't see any rational, practical reasons for Avebury or its ditches. Ditto Silbury Hill. Ditto the lengths of long barrows. Ditto cursus. That is because I am bound to look at prehistoric society through the wrong end of a dim and murky telescope blinded by modern pre-conceptions. It does seem to me though, that when people build irrational things they do so either for religious reasons (churches, temples, shrines) or for reasons of personal kingly/presidential vanity (palaces) - or a combination of both (pyramids). I reckon those Neolithic guys were religious nuts. All that farming, with ergot in the wheat, drove them barking mad ;-)