Why is lithium so unevenly distributed?
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.