Eh, I don't think it's too surprising, really. Hobbits basically live in an idyllic paradise, and those few who want a bit of adventure can find plenty (for a hobbit, at least) near the outskirts of the Shire. The outside world is in far worse shape, meaning there
is a barrier - with the exception of the brief period of time between the end of
The Hobbit and Frodo's departure in
Fellowship of the Rings, there are monsters lurking about if you go too far afield, so what right-minded hobbit is going to leave? For those very few who do, Gondor is
pretty far away (by my measurement, the closest border of Gondor is over 600 miles away from Hobbiton, and that's in a straight line).
Now, why hobbits only survived in the Shire* isn't ever addressed to my knowledge, nor why the Rangers apparently thought this small, insignificant community was worth dedicating so much manpower to protecting. Honestly, with how resilient and scrappy they are shown to be - once you get them out of their comfortable homes and give them some incentive, anyway - I would have expected more to have survived.
*Smeagol's folk seemed to live outside of the Shire, and were clearly some sort of hobbit, but I don't think we ever find out just how old Gollum is - his people may well no longer exist, and if so probably haven't existed for a very long time. Or, who knows, maybe they're the ancient ancestors of the Brandybucks.