The biggest source of style pages is event rewards, so ZOS tries to encourage us to trade or sell our extras to help our fellow participants who may not be so lucky with RNG. Normally, that's fine, except when the playerbase gets flooded with Glenmoril or Prophet pages and then you practically can't give them away.
ZOS probably won't change it, since it has the desired effect in most cases and they'd rather have you discarding the overly plentiful pages than hoarding up the rarer ones.
As for why they don't stack while motifs do, I can't think of any container that does stack. For all that Style Pages function similarly, the game seems to handle them more akin to runeboxes than consumables like motifs judging by how they are sorted in the guild traders.