It's not that saints are 1-1 copies of pagan gods, it's that Catholicism and Christianism has adopted pagan practices in many places has adopted pagan imagery and ritual. Even the popular image of God like depicted on the Sistine Chapel stems from Zeus/Jupiter, early images of Jesus were similar to Apollo, and so on.
In the cases of "classical" religions, there hasn't been much more than aesthetics and ritual though. While surely there are some interesting connections, pagan beliefs, theology and worldviews were much different from the Christian ones. The representation of God might be similar to Zeus, but the conception and role of God is entirely different. I dare say there wasn't a relevant syncretism between Christianity and Hellenic religion, rather Christianity replaced it with only some aesthetic and cultural remnants
But there are more interesting cases, in Latin America in particular. To add to what other people said, I've seen some rituals to the Pachamama as well as other Latin American folk saints, and often they have very little in common to the European concept of Catholicism except for names and feast days. For the Andean peoples, celebrating the Pachamama and the Virgin Mary is the same thing and they see no contradiction on it. I think the particularities of those beliefs go beyond simply aesthetics into actual syncretism, and are worthy of study.