or maybe, their robots are really never done and they will keep adding to it?
This is probably closest to the truth. More charitably, many hobby robots are "experiments" and are never really MEANT to be "done."
when done, take the chip out of the arduino, insert it into a bare minimum PCB - bare as in only the atmega on a socket, the resonator, hookup or the actual +5V regulator, headers for the servos and sensors, etc. - live test and if all goes well, "go live".
Keep in mind that having a "bare minimum" PCB made is likely to cost more than the arduino module. If the "development platform" works, and is cheaper than "going live", AND you can take the pieces off and reuse them for something else if you need too, what was the advantage of making a "pure" finalized "product" ?