The ethics of dog breeding: Help me to understand dog breeding monopoly

As I am searching for a pup myself, I am noticing that most breeders had a no-breeding clause in their agreement. This is fine with me as I'm not looking for one to breed and have a good understanding of how non-neutered dogs are impacted health and mood-wise. What I am curious about is the honest reasons behind the agreement not to breed. I get that breeders may want to prevent puppy mills but I can't help but wonder if they are just wanting to create monopoly on selling puppies. Can someone explain what the true reasons behind this?