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 🙄.

not mine

I think some of the humans had the curiousity, which lead them to have such imaginations.

If this is all there is, then that sucks.

Maybe we all hope that the stakes aren't that high.

If life is abundant in the universe then maybe somewhere there is a better place with better beings who will make better choices.

Either now, or in the future.

Because we've never seen anything like it before.

Grass is greener on the other side, same concept