Same reason he managed to shoot himself with a harpoon. Seems like the brain damage was there all along; getting shot in the brain didn't make things worse.
I thought this was an easy puzzle. But then I saw the comments and realized that some people aren't reading it correctly. That must be what makes it puzzling.
https://www.youtube.com/watch?v=mpaEYv8teGY
Bad Chewie, M.