The function of the FC is to "funnel" the bullet into the bore. It will reduce distortion of the bullet, particularly if/when things are not a PERFECT line-to-line fit. I like 11 degrees, cut all my own, my Pa's and perform the service for friends from time to time.
Why do mfgrs not address???
Better to ask them, but I'll guess that it costs money (more finish time, work & tooling).
Regards,
Slim