Why are spren how they are and why do they only appear on Roshar?

[the_bros11_2]

Why are spren how they are? I mean, I don't associate a weird long-tongued hopping thing with anticipation.

And why do they only appear on Roshar?  (unless Seons are spren)

[SteelShaper]

First, by my understanding, spren and seons [and skaze] are all Splinters. So they are technically the same thing.  Second, the main difference is that on Roshar, the Shards had more influence over their Splinters, and some of the Splinters/spren [or maybe all of them] are a combination of the two Shards.  There are so many combinations that the spren evolved their own ecosystem.  The form the spren manifests in either the cognitive realm or the physical realm doesn't so much relate to how they are perceived or what they are attracted to, it's how the power that became the spren evolved according to whatever weird natural laws govern them.  Some combinations are more drawn to certain emotions/things, and other combinations have evolved/been created to be sentient.  As for the other ways that human/sentient perception shapes Shadesmar and the spren, Sylphrena says that some of the older spren even have 4 genders, "because humans didn't think of those." So it may also depend on what species was predominant at the time a given type of spren evolved.

Doubling back for a moment to Shardic involvement, the Splinters/spren on Roshar were [mostly, though not all] created deliberately.  Honor and Cultivation allowed some of their power to dribble off to make the spren.  On Sel, the Splinters [Seons and Skaze] came about only after Odium Splintered Aona [Devotion] and Skai [Dominion].  It's been theorized and hinted that some artificial force or mechanism, as yet unknown to us, then helped to bring the Splinters to self-awareness.

Also, I think some of the types of spren predate the Shattering of Adonalsium, whom I believe Brandon has said specifically created Roshar, unlike some of the worlds that already existed but were altered by its power or the power of the Shards, and that is why on Roshar we get those visible manifestations of the spren in the physical realm, because there is some kind of bleed-through from the cognitive realm.

[Honorless]

Spren are Splinters, and we do see Splinters on other Shardworlds. Seons and Skaze are the most spren-like (canonical) entities that we've seen so far. But the spren of Roshar aren't just Splinters, they're Cognitive manifestations of ideas¹, this is by Adonalsium's design² and might have something to do with Roshar's Cognitive Realm being a nexus hub³.

1: https://wob.coppermind.net/adv_search/?query=spren%2BPlatonic

2: https://wob.coppermind.net/adv_search/?query=Adonalsium%2BRoshar

3: https://wob.coppermind.net/adv_search/?query=Roshar%2Bnexus&date_from=1998-04-10&date_to=2020-06-27&speaker=&ordering=rank