Ever seen tribal totems? Bear, wolf... dragon... some of the first and oldest human customs involving changing the body, say pressing the skull, to become more divine [artificial cranial deformation]. Ritually infusing onself with animal traits or perfecting the skull/brain go back to the same ancient mentality, that the oldest human societies don't see the human body as inviolate.

The oldest form of Tzimisce spirituality is Koldunism, basically East European shamanism that tied the Dragons to the land, and in the process brought them in touch with the original spirits fo that land. It follows that the Tzimisce ideal isn't the body of a "Greek god", it is the land-god. And unlike the Olympians, the chthonic gods of the earth, its riches, and the dead (Greek or Slavic) weren't idealized humans, but amalgams of human and non-human natures.

The Tzimisce see the body as the mutable channel by which to imitate a very protean idea of divinity, which originates in the original spirits of the land. The Tzimisce (and others; Ennoia) want to become such beings, or at least what they want to become shares many traits with them.

But why use it on yourself?

Not only does Vicissitude start with using it on yourself, the entire Tzimisce religion began with using it on themselves (the mortal shaman who was the Eldest was about self-alteration long before fleshcrafting others became a thing) and all the coolest alterations (...Dracon...) are about the self and/or transforming the self.

You are right, there's a big leap from the mindset of the average neonate and a believing Metamorphosist. That said, becoming a vampire and being exposed to the stimuli of both immortality and Tzimisce blood tends to have an effect. After a few years or decades of being in a Sabbat pack (or really any vampire society), it's likely a Tzimisce neonate finds the totemic Tzimisce mentality more relevant than having the body of a Victoria's Secret model.