Cutting or polishing granite is not the doubt but to reproduce them in exact size precisely like machining is a challenge in those days, notice sculptures are irregular in shape not perfect flatness even at the concave and gutter like inward shape, since granite is so damn hard, why make such a structure that has no appearance purpose?

There is only two answers given,
1. Alien intervention
2. lost civilization who could be rather advanced, even with such advancement, they extinct or left the Earth.

Stone isn't the only sign that led to alien, crop circle and psychic and the paranormal too have a long history in human mind but since human can manifest anything, it is deemed unreliable.