why can't ordinary people walk on the runway?

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i have seen the most beautiful and most skinny women walk down on the runway to model the works of the popular designers. i was just wondering why ordinary people, which we see everyday, can't walk down the runway to model these clothes. i don't intend to offend anybody, but people around that we see everyday are not always as beautiful and skinny as the models in the runway. i think clothes are made to enhance the beauty of the one wearing it. so don't you think it would be better it we could see how an ordinary person, who would probably buy the dress, would look when they wear the dress? what do you guys think about this?