It's the second most common element in the universe.

Lithium isn't in the top 10 most abundant chemical elements in the universe ( https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elem... ). The most abundant, of course, is hydrogen, making up close to 74% of all the baryonic mass, followed by helium ~24%. Lithium can be fused into heavier elements and the conversion happens efficiently in stellar environments where it is produced.

For every 10^6 atoms of silicon, there are only 10^3 atoms of lithium in the Earth's crust ( https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth... ).