In Ratchet & Clank it's always taken for granted that all the robots think and feel. Maybe at some point a group of robots decided to form a robot haven on an uninhabited planet. It would explain the lack of anything biological and I could totally see Ratchet and Clank crash landing in the middle of Robot City.
Robots is a metaphor for the American Health System
If you're rich enough, you can afford upgrades, which can be classed as plastic surgery. You can look just like everybody else. If you get sick, you can afford to be repaired. If you're poor, you find somebody who will do it for free (Medicare), or you drop dead. The fancy new upgrades came with 'cup holders', but apart from being shiny they didn't really do anything spare parts couldn't. But they made a huge profit for the big companies. The people who could afford it were being ripped off, and those who couldn't were left to die.
Robots is a allegory for the Holocaust
Look closely to Ratched and the Robots that can afford the upgrades... Do they have something similar to a kind of Superior Race? Yes they do, and Ratched wants to get away with the old, rusty robots. Gee, does that sound familiar?
There were humans.
The story takes place After the End
, and humanity, along with most/all other animals are completely extinct. The end might not even have been violent; the humans might've simply stopped reproducing after putting all their energy into creating robots (like in Futurama
, the proposed apocalypse that would happen if humans took robots as lovers). In any case, end happened a long time ago, and it's likely no living robot even met a human.
This movie takes place in 01
The machines built their own cities after being pushed out by humanity. Why no war? The movie takes place during relatively short peace time between the humans and machines, and these are "civillian" robots who care nothing for the war. Explains why there seems to be no real law or reprecussions to stop murders (a business CEO can apparently order genocide of the lower class, and while the lower class disagrees with this move, nobody thinks to get the equivelent of the police involved).
Piper and Fender aren't biologically related.
They could be stepsiblings, or one of them was adopted from a "robot orphanage" (possibly Piper). Either way, neither of them share physical similarities with the other.