Came across pool #19688 just now. Couldn't find any forum discussion for it, didn't really have a description, and just looked like another pointless pool randomly created by a member level user. So, I deleted it. Only afterwards did I realize the description, bare as it was, had been added by a moderator.
The thing is, traditionally we haven't consistently made pools like this. The more popular copyrights (e.g. Arknights, Fate/Grand Order, Granblue Fantasy, previously Azur Lane, Touhou, and any other non-gacha copyright we have official art of) have never had pools for their character portraits. FGO does have pool #11271 for Riyo's April Fool's character portraits, but there isn't one for the standard portraits. Doing some brief looking around, there are a few other pools like this for smaller copyrights, but like the Azur Lane pool, were seemingly created by a single user with no prior discussion: pool #19163, pool #19172. Umamusume and Mahjong Soul also have them, the only larger names I saw (is Mahjong Soul big name? I never hear about it.)
Currently, whether or not a given copyright has one of these pools is just arbitrarily up to the whims of the franchise's fans. Should we start more consistently creating pools like this? I really don't think the smaller copyrights really need it, but many of the larger ones could benefit (despite having literally just deleted one of them), with how much promotional art and official comics flood their official art searches, though anyone above member can just subtract those from the search. Personally, though, I would just visit a copyright's largest wiki if I wanted an easy perusal of its character portraits; the Azur Lane wiki actually has more than you can find here.