More like Starfleet is run by imbeciles. They seriously didn't think it was worth warning the fleet about, even after the "Regeneration" incident and the loss of the Hansens?
And why didn't Starfleet seem to be aware that the Borg assimilated lifeforms as well as technology in "The Best of Both Worlds", when Captain Archer found that out over two hundred years ago?
Because Regeneration was written 12 years after The Best of Both Worlds.
Nothing is planned out in advance.