The robots were created by a higher civilization as a terraforming tool.
They terraform all the planets they encounter to the living conditions of their creators, which happen to be hellish to you.
They are tasked to terraform all the planets they see, and you are like a pest to their creators, so any obstruction from you part, such as living in the planet they are going to terraform, is met with violence.
There robots look like tools and construction machiery, because thats what they are. They dont have humanoid shapes, most of them look like vehicles. Also, they dont have weapons, they attack you with powerful tools (their creators are so powerful that just their tools are about as much powerful as your weapons of war).