Why does the idea of life on other planets capture human imagination?

Multiple reasons. IMO the biggest being man’s search for something bigger than ourselves, the discovery of a simple lichen would mean life (complex or otherwise) would flourish everywhere in the universe. The implications would shake us to our core, philosophically. I would hope it could inspire a new age for humanity, all of us working together to develop ways to further explore the vast reaches and hopefully make “first contact”. At the least it might stop the constant fighting over whose “god” is better 🙄.