"Quality control" doesn't tell you what standard they have for acceptable, just that such a standard exists.

I've heard that it is quite straightforward with modern tools to make new violins which pass a blind test against a Stradivarius — I wouldn't know myself — but that doesn't mean those with a target price of £500 are actually made to this standard.

It's closer to a difference between mass retail and tailor fitted clothes. Big vendors have great quality control too, but it's not the robustness of the stitch or uniformity of color that's important here.

I think there is some survivorship bias at work. An instrument that feels and plays great was just more likely to survive several hundred years.

I could see differences due to materials. Wood is not uniform product, so picking the right section for right part might do something minor. Metal is metal and we get very exact and uniform properties with modern manufacturing.

Now maybe we could question why have we not come up with anything better for these wood instruments? Like some mixture of plastic?

Carbon fiber seems to work reasonably well for guitars, I don't own one but I've tried them out and I think they're perfectly fine.

You must be right, those thousands of professional violin players who've been playing since kindergarten are just deluding themselves.