Theory about time frame
it is my theory that this show is taking place in the future. titans destroyed most of humanity and those that are alive live behind the walls technology is closely guarded the government keeps the knowledge on how the 3D gear works so what about keeping past technology secret.
we know there are religious people in the story so what if they government has full knowledge of past technology but view it as sacrilegious to use such technologies(ie planes,cars, electricity etc etc)
Also i think that there are other nations still in existence behind there own walls