If you look at the Colosseum, it looks the way it is because it was willfully destroyed and used for construction material over centuries: when Rome became the seat of the Papacy, the Colosseum was remembered as a place where the early martyrs were slaughtered by wild beasts, not as a nice building to be preserved for tourists.

On the other hand many Roman bridges throughout Europe still stand and some are still in use (yes even by cars). But the really amazing case is the Pantheon, that was turned into a church early on and preserved and restored for almost 2000 years. It's true that not all we see is original, but the structural engineering is.