They are hybrids - they don't breed true. Just like hybrid seeds for plants or the breeding to make mules, they are the end cross of a mixed selection for breeding. The parent generations aren't going to look or act or gain like the cornish X, but will carry their own attributes. When crossed, the resulting chicks carry the correct mix of genes for fast growth, high feed conversion, and the carcass traits required.
Breeding is not done from the resulting stock, but from the grandparent/parent stock. The cornish X aren't expected to reproduce.
Think if you're breeding mules. You can't breed two mules together to make another mule. You have to breed a horse with a donkey to get your mule.
Breeding Cornish X isn't how the breed is propagated, the breeding of two specific lines of chickens (non-Cornish X) gets you the cornish X.
Also, they aren't pitiful creatures that need sympathy - they are pretty amusing little buggers. Otherwise I don't think they would be so popular with folks raising them at home. They require attention and care to grow well, but they are worth it. I enjoy raising them as much as I do other livestock.