Perhaps because some things are unethical.

Broadheads kill by hemoragic shock. This was explained very well by a surgeon Dr Baumgartner several years ago. I wish I could find his article.
The bottom line was that it was absolutely necessaryfor the broadhead to be razor sharp. Do people take animals with dull heads? Absolutely! Do people lose animals with sharp heads? Of course. But just as HNI_Christine said, we should control every variable we can. Broadhead sharpness is one of the easiest.
I did find this article. In it the last paragraph speaks to "sharpness"
http://www.bowhunters.org.au/Broadhead%20Study%20Part%201.pdf

Whether you sharpen or change your blades, we owe it to the animals we hunt to kill them as quickly and humanely as possible. Anything less is well...unethical.