Rare doesn't always equate to a good playing game. In many cases the company in question field tested the samples/prototype games prior to production and found it didn't resonate with the playing public so they never made it to full scale production. With the change in paradigm from playing pinballs to simply collecting them like playing cards or some kind of quasi trophy piece these doorstops are now considered desirable. This is also why the LE business model from Stern has hooked so many suckerfish.
"Rare" is literally low production run, but can be games that are highly sought after, and not easily obtained. Safecracker comes to mind.
Or you could just stumble upon one dumped outside of a pub and pick it up for free.
