It has nothing to do with "mutts," but performance selection. Dogs that are bred and tested to work may not be perfect and flawless...but they are VASTLY superior in health and ability to dogs traditionally bred for show or non-working purposes.
You know why working dogs are more able and healthier??? Because if the dog has a problem it isn't excused but instead removed from the breeding stock. Show breeders however don't remove a dog that is unhealthy if it looks good. In fact, many breeds are often bred to look pathetic on purpose...take the Neapolitan Mastiff, Sharpea, or English Bulldog...distorted on purpose. And, it is no wonder those breeds top the list with problems.