Selenium is actually not very abundant. It's the 69th most abundant element in the Earth's continental crust, at an average concentration of about 50 ppb. It's less abundant than silver and only 10x more abundant than platinum.
World production for 2022: 3,200 tonnes
World reserves: 81,000 tonnes
There's not a whole lot available. It occurs at 0.5 to 12 ppm in coal, so large amounts of coal ash might be a future resource. It's not economic to extract that now, vs. current secondary extraction from copper anode slimes.
https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth...