Well I think there's two mechanisms at work - 1. everyone tends to slip into over-defending their pet theories and 2. it's not a good career move for archaeologists to rock the boat. And so the cow gains gravitas. Human beings, eh?
As for "where the weight of evidence lies?" that's a philosophical point. Does "some evidence" outweigh "no evidence" Numerically yes, but it's a dangerous rule on which to base an opinion as it might be wrong!