Is there any reason why the robots were buried and forgotten?
While nothing is concrete, it appears that the Sheikah either A: Became outcasts and were feared by the Hylian people or B: Adhered to the royal family's warning, and forgot their creations and left them to be buried.
"Eventually, however, people began to fear Sheikah technology being misused, causing the Sheikah to either become outcasts, or follow the Royal Family's warnings by burying their technology and living much simpler lives."
(The quote came from here)