OK, how else do you enforce rarity?
Let's take my current setting as an example, so I can't be accused of picking on anyone else.
In that setting Gnomes are a rather rare sight - there's only a few tens of thousands of them left worldwide, and many of those are widely scattered. Gnomes are, however, still a PC-playable species in my game. To reconcile the low odds of ever even meeting a Gnome with their PC-playable status, I want to somehow make them rare - but still available - as PCs, so as not to end up with a disproportionate number of them in adventuring parties.
It should be noted also that for a few classes Gnomes are a very optimal/powerful choice - their generally high Con, high Dex, and reasonable Int lends itself extremely well to low-hit-die back-line classes such as Thieves and Mages.
How would you mechanically enforce that rarity?