The Assimilation of Myth, in a Culture ... is a Protracted Process of Cultivation, and Adaptation of the Foreign, to, the Local.
Romans, Believed, like most Ignorants Believe ... that, they Knew all, there was to Knowledge; but none the less, adapted the Greek Myth ... Pantheon ... for their own conveniences ... thus, their adaptation stretches over a long period of time.
Greek Myth's Adoption, was in the Naturalization, of Greek Metaphysics, by Romans. It can be said that the adaptation Period spans over a Millinneium; circa 500 B.C to 400 A.D.
As Regards the Why ... Well, all Ignorants, think, that the Assimilated, in reality, is their own, because, they have no benchmark, to distinguish the Validity of their own, from other current beliefs.
Its same, as the Myth of UFOs, present eversince the 14th or 15th century A.D ... but only after about 500 years of Cultivation, and Assimilation, while Science replaced Alchemy ... that the UFO has become, a world wide Myth.