>humans about to reach industrial revolution and then significantly damage the Earth's ecosystems
>aliens want to save Earth's biodiversity, including humans
>aliens disperse nanomachines to form titan bodies out of thin air around human population centers
>titans engineered to kill humans specifically but don't significantly affect any other aspect of Earth's environments
>aliens create the three walls to maintain a healthy human population size
This is the only non-magic explanation for AoT I can think of.