Because they don’t know how people in the past built things and doubt they were smart enough or could have pulled it off. Humanity hasn’t evolved intellectually anymore in the past thousands of years, as a society sure thanks to building off prior knowledge, but a lot of knowledge was lost.

If you think about how we got people to the moon before the personal pc, it’s damn impressive. Or how they used pigeons in WW2 to help rockets hit targets. Or reading about the Bagdad battery, from thousands of years ago.

Our ancestors were just as smart as us, they just have the collected knowledge from enough generations to build upon to advance like we have today.