Their sacredness is not related to metaphysics, it is not originating from their existence itself. As you said it is just because one certain spider is believed

Their sacredness is not related to metaphysics, it is not originating from their existence itself. As you said it is just because one certain spider is believed to do an important thing. For example AFAIK spiders are not used as figures in Islamic Culture, they don't have semiotic meaning. So Ostrich eggs being anti-spider doesn't necessarily mean they are evil/unholy.