I’m not a Jew but a few I know who are non-religious told me it is the basis of their cultural and family identity. There are blurry lines around religion, philosophy, and identity. And not just with Jews. There are quite a few Buddhists who practice it as a philosophy but no a religion, and countless other examples.

There’s a guy on YouTube (Esoterica channel) who is a post-theist Jew who (occasionally) talks about this. He observes Jewish traditions and so forth, but he doesn’t think God is real, etc. Fascinating channel BTW.

Anyway, I understand your confusion and I hope this doesn’t sound condescending, I don’t mean to be. I think you’re overthinking it a little bit. I was in the same place years ago.

Hopefully a Jewish person answers you and explains it better than I did.

To give a super silly and reductive example: imagine you belong to a Star Wars fan club that you LOVE being a part of. Some of the members think the Force is real but you do not.