Because the seeds made will likely not resemble the desired cultivar. Clone only strains are either renamed from a purchased batch of seeds, or the result of some level of hybridization or inbreeding. Sometimes someone cracks some beans and finds the ultimate keeper, which has so many desirable traits it gets singled out for propagation and eventualy renamed and passed on. Other times a male of one type is crossed with an unrelated mother and an amazing seed or two are made that shine. A name is thought up and yada yada. Yet again a specific breeding project is undertook and produces magic. Greed or fear makes said breeder horde the seeds and some clones are leaked or handed out. And many other scenarios.
Yet few of these roads will lead to a female, that when feminized and used to pollinate itself, will result in seeds that will earn anything but ire and bad rep from those who pay for "true UK cheese".