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It's because they're a documentation mess caught in a release/stable discussion that I don't understand.
Perhaps that changed in the last year, haven't looked into them again since then - but unless there's a trigger (new server setup or official release for example) I simply don't have the time and energy to invest.
Because I don't understand them. Last time I looked, the documentation on flakes was minimal to nonexistent. What documentation I did find was all 3rd party and conflicted with each other.
Granted, that was a 2ish years ago, but I haven't needed to change anything major, and nothing has broken. My current setup just works.