Why do spren exist on Roshar but not other planets in the Cosmere? We know that 'everything has a spren' (WoK) and also 'we know about Windows; what is and isn't a window takes on... meaning in the Spiritual Realm. Takes on life, after a fashion.' (ES) So across the Cosmere every object and idea has a 'spren' but only on Roshar are these ideas able to present themselves in the Physical Realm. Is the barrier between realms thinner somehow? Or is the system so saturated with Investiture?
I think the Spiritual and Cognitive realms appear differently on different planets, and that’s why? So yes, I think spren would only appear as what we think of as “spren” on Roshar. There many be different ways the Spiritual and Cognitive realms manifest in the Physical on different planets…and those may or may not be equivalent.
We’re definitely seeing different appearances of our realms as we world-hop (SH and Elantris stuff come to mind immediately). I’m shaky on my Realmatic Theory, though, so if anyone else wants to cut in please do!