1. To build a robot you have to mine raw materials and refine them and assemble them and program them. This takes a while, but to build a human worker you need to feed, house, clothe and educate them for 18+ years.
2. The human can only reliably work 8 hours a day. During that time you need to provide ample overhead lighting, and heat or cool thousands or millions of cubic feet of air to a comfortable temperature (this alone takes a tremendous amount of energy). Outside of those 8 hours, the human consumes an entirely separate structure that also has to be lit and heated and cooled. Further, the human has to be fueled with food that has to be grown out of the ground or via livestock that has to be carefully tended.
The robot, on the other hand, can stay in one place it's entire life. Doesn't need much light or temperature control. It runs on electricity which can be easily generated compared to growing a cow or a field of corn. Eventually it may even be solar powered. Aside from that it will work 24 hours a day without complaint (with some occasional maintenance).
So, if you replace 100 humans with 100 robots, you might use the extra resources of 1 additional humans. Now, if out of those 100 humans even 1 of them does something productive that can't be done by a robot, you've got a net win. I think the resource consumption advantage is tilted heavily towards robots.