This is a small complement to David's and Scott's answers
As usual Wikipedia's page on Lithium contains useful information :
Both natural isotopes have anomalously low nuclear binding energy per nucleon compared to the next lighter and heavier elements, helium and beryllium, which means that alone among stable light elements, lithium can produce net energy through nuclear fission. The two lithium nuclei have lower binding energies per nucleon than any other stable compound nuclides other than deuterium, and helium-3. As a result of this, though very light in atomic weight, lithium is less common in the solar system than 25 of the first 32 chemical elements.
⁷Li is one of the primordial elements (or, more properly, primordial nuclides) produced in Big Bang nucleosynthesis. A small amount of both ⁶Li and ⁷Li are produced in stars, but are thought to be burned as fast as produced. Additional small amounts of lithium of both ⁶Li and ⁷Li may be generated from solar wind, cosmic rays hitting heavier atoms, and from early solar system ⁷Be and ¹⁰Be radioactive decay.
So, basically, Lithium is (barely) produced as David Zaslavsky told you in his answer, and the reason the production is low because Lithium is barely stable.
But as @Scott Carnahan tells in his answer, the notion of lithium scarcity is linked with its repartition on earth. And the reason it is difficult to obtain is ultimately its high chemical reactivity, which means that it is basically diluted everywhere, and is rarely concentrated in easy to mine deposits. On the same wikipedia page as above, they say :
Although lithium is widely distributed on Earth, it does not naturally occur in elemental form due to its high reactivity.
According to the Handbook of Lithium and Natural Calcium, "Lithium is a comparatively rare element, although it is found in many rocks and some brines, but always in very low concentrations. There are a fairly large number of both lithium mineral and brine deposits but only comparatively a few of them are of actual or potential commercial value. Many are very small, others are too low in grade."