So I've beeen thinking. By this point, the Culture has done the following:
-Scanned a bunch of imperial planets, getting a whole LOT of statistical data
-Probably scanned the brains of a bunch of psychic species (both animal and sentient), and psychic individuals, from minor species with specific abilities (and there are a LOT), to weirdboyz, to eldar, to nicassar, to sanctioned psykers and astropaths and Librarians. They might have even scanned a Primarch or something
-Scanned a bunch of the Mechanicum data for all of those Genetors that always regularly do experiments on psykers and stuff all the time, but which never goes anywhere due to Mechanicum Shenanigans
-Gotten a decent idea of what sorts of 'things happening in a person's life' tends to correlate with psychic ability (like being the seventh son of a seventh son and other circumstances that match patterns, based on imperial and other civilization's data archives, and brute force statistical and data mining, so have a pretty good idea of how nature+nuture+circumstance contributes to a powerful psyker with appropriate levels of control and safety
-Gotten a decent idea of what brain architectures seem to be tasty to Chaos, and which ones specifically don't
-Gotten a decent idea of what organic components, brain architectures, genetic architectures, mental capabilities, extra organs, lifestyles, and life circumstances tend to correlate with powerful psychic capabilities AND the ability to use them relatively safely and control them relatively well, by surveying a whole lot of psychic species


With this, they probably could just BUILD an organic psyker and set up all the circumstances to dramatically increase the likelihood that this person is a powerful, controlled, high willpower, relatively safe, relatively versatile psyker. So either why don't they, or at least ask their Eldar allies 'What would happen if we do this? We are lagging behind on psychic and warp understanding, and feel we have to take more extreme measures.'