Probably because a very good shooter can shoot very good groups, a not so talented shooter buys the rifle and can only manage groups twice the size with it, and then posts everywhere that he was cheated. Why open yourself up to that?
If you're on Facebook you would be very familiar with people that constantly post that one photo of that one amazingly tight group they shot once, claiming that's how good they or their rifle shoots
I have shot a single 5rd 50m group of 6.5mm with one of my Ruger .22's, and a few that are sub-8mm, but those are all anomalies out of many thousands of rounds. I was actually asked by a dealer when I was collecting a pile of ammo what I thought of my Rugers. As I had shot that group just prior I told him about it. He pretty much told me I was a liar, even though I was not claiming that to be anywhere near the norm. No skin off my nose whether somebody believes me or not. Anybody that has done any shooting knows how the statistics of grouping bullets works, another 100,000rds down the tube and I might even fluke the holy grail (to some) of a neat round .223" hole for five rounds, but it won't be anything to do with my ability
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I guess I thought that - regardless of shooter - showcasing what the rifle was capable of might help sell a rifle. I know if I was choosing between two...and there was no discernible difference, a proven target might be the difference.
Based on what your saying BR. - might as well take the proving targets, that are sent out with some rifles, and Make paper aeroplanes.