But we also can't just 'poof' imagine something outside of our realm of existence/experience. If a spiritual world does not exist in some form or another, then I don't see how we could have conceived of it to begin with.
Tec, using that logic, God also CAN'T exist because otherwise people couldn't conceive of a universe where he doesn't. Aliens also MUST exist because we can conceive of them. And they must also NOT exist for the reason I state above about god. Mermaids, ditto. Sasquatch, ditto.