The stellar processes that result in stable lithium are rare, like supernovas.[1] Another factor in its rarity as a material is that while it's present in a lot of things, it's not present in quantities for economical recovery.
Ahh, thanks! So if I understand correctly, it's because its stable isotopes can only be produced in high quantities through fission rather than fusion (otherwise they get destroyed in the same processes).
1. https://www.forbes.com/sites/startswithabang/2019/11/14/this...