For decades, the general aviation industry has struggled with finding a replacement for leaded avgas without success. The biggest driver of this failure is that there's no reason to do so because the industry has been given an exemption to continue using lead.
In this two-part series, AVweb's Paul Bertorelli explains how industry inertia and bureaucratic foot dragging killed efforts to eliminate lead from aviation fuel. See Part 2 here. https://www.youtube.com/watch?v=Mvse4Xhzwuk
For an excellent overview on leaded fuel in the wider world, see Veritasium's excellent video on the topic.
https://www.youtube.com/watch?v=IV3dnLzthDA