Why is lithium so unevenly distributed? According to the US Geological Survey, nearly all of the world's lithium reserves are in Chile/Argentina, China, and Australia. Why is this? Lithium seems to be mostly harvested from brine pools, so are these just the areas where lithium can be economically harvested, or does it represent an uneven distribution in Earth's crust?
The term "reserves" doesn't mean what most people think it does.  A mineral deposit is simply a concentration of a mineral; a reserve is a deposit that is both known and economical to extract.  As an example, oil shale and tar sands have both been known for decades, but only recently became oil reserves as improving technology and rising prices made it practical to extract the crude oil from them.

Chile, Argentina, Australia, and China have quantified their deposits, and local economic/geologic conditions make it profitable to extract those deposits, so they've got most of the world's reserves.