*le shrug* Basically just how the games are designed, I guess it's supposed to add a sense of
pride and accomplishmentdiscovery and gamble or something? Mostly just becomes a damn hassle real fast and is one of the prime reasons I'm a shameless abuser of the save editors.
Mind you lore-wise it's fitting for the CoCs since in the first one it's basically the bizarro effects of the demonic corruption of the world at work and in the second no less majikal alchemical concoctions and whatnot, neither of which is anything like an exact science. Rather less excusable in TiTS where the majority of the things are supposed to be commercial consumer stuff - even if the Big Pharma megacorps making that shit don't have to pay more than lip service to medical safety standards in proper dystopian cyberpunk style you'd still expect them to package a pretty exhaustive list of the effects simply so prospective buyers actually know what to grab off the shelf... (granted, might be a bit of a pain to realize in UI terms.)
And don't get me started on the more ridiculous lootables that make about as much sense as eating a Big Mac somehow genetically transforming you into a phat Murrican rather than whatever the beef in it came from; looking at you, Zil Rations et al.