I'm not a photographer so please excuse what is probably a naïve question: why would the lenses vary so much? Does it mean that photographers in the UK have on

I'm not a photographer so please excuse what is probably a naïve question: why would the lenses vary so much? Does it mean that photographers in the UK have only a one in three chance of getting a good lens each time they buy? If the quality is so variable wouldn't this generate a bad reputation and thus reduce the sales of the brands that were poor and increase the sales of those that were better?