It might be a form of self - expression. The 'stupid' in 'stupid poem books' could be a term of endearment, like they like these books despite their flaws. By putting it on their snap story, they are inviting others to start a conversation about it, perhaps about the deep or silly things they found in the poems.
Maybe they want to show their love for literature in a fun or ironic way. It could also be that they just found a really interesting or unique poem in the book and want to share that discovery with their friends on snap.
Well, it could be part of a trend. Maybe there's a new style of sharing 'old - fashioned' or 'quirky' things on snap, and these girls are jumping on the bandwagon. They might also be trying to create a certain image for themselves, like being deep - thinking or having a unique taste in reading materials.