Because they expect the buyer to use factory ammo,, and their factory ammo to boot,,
And expect the buyer to be content with SAMMI Specs including mediocre accuracy out of the box.
As soon as you modify anything you in effect void the warrenty even if it improves accuracy.
Face it, we reload, that's why we're on this forum, modifying guns and ammo happens to be our bag, we enjoy finer more reliable accuracy, yet we still represent a smaller percentage of the gun buyers out there.
At least that's my take on it.