Most likely, at first, all slave characters are of course "untrained", with some minimal set of characteristics, so that you can start with training the simplest slaves. But if they remain like that forever, then as the plot develops, when you need conditional "performers, courtesans, brothel slaves or breeding slaves", you will not be able to find them on the market - there they will all be - "initially stupid and ugly". That is why dev made a gradual increase in their characteristics ("shadow", not from the development of the plot, but simply increasing), so that you can eventually pick up the necessary ones with the minimum required characteristics for a certain specific training. But are playing you and other players and dev cannot know when exactly you or someone else will need to start training more serious characters. Perhaps that is why their characteristics grow faster than you would like, and, for example, another player does not have time for "400 game days" and such a speed of "self-training" of slave characters is quite comfortable for him.. By the way, I don't know (I haven't checked) whether it is possible to subsequently "train" those who are initially "smart and beautiful" from the market for simpler duties as the game progresses, and if it is not possible, then this is really an omission.