Culture Minds have a quirk of their build that made them what they are - it's not the norm, though there are other Mind-like AIs that joined later. Building super intelligent unfettered AI has a known end condition where said AI at worst might steamroll local area then pass onto higher plane of existence or however you decide to call the process of Sublimation.
Because a super intelligent AI quickly can reach that point, and with resources available to many Involveds, finds cheaper and more interesting methods to reach there than disassembling biological life.