The process might also be simpler/less Investiture intensive for Cognitive entities. Cultivationspren apparently manifest a lot of souls of inanimate objects as their version of 'gardening', as we are told by both Wyndle and the Honorspren abroad the Honor's Path.
It might also simply be due to different Shardworlds, different Shards who've Invested in that Shardworld, different forms of Investiture on the Shardworld or even how Investiture saturated they are, or distance between the manifestation & the original object, or all of the above.
Besides, have we actually seen humans manifest a soul in the Cognitive in the same manner? Shallan created replicas of objects in the Physical made of the beads of the Rosharan subastral's (in)version of land, which judging by the fact that Jasnah also used the same method, is probably something granted by the Surge of Transformation.