Why do companies that make dog food add carcinogens to dog food?

Companies of inexpensive dog food will use the cheapest source of grain. This has created hikes in dog deaths every few years from contaminated food. Cheaper dog food will be loaded with numerous additives with many being carcinogens. Chemicals that cause cancer. It is thought that even small amounts of Aflatoxin fed over the years will create more susceptibility to cancer. Aflatoxin is just one of the unhealthy components of dry dog food. Cancer is the No. 1 disease to shorten the life of your pet and the ASPCA has given the figure that 60% of all dogs in the U.S. over the age of 6 will be diagnosed with cancer in their lifetime. It is also the number 1 killer of dogs over the age of 2. 6 million dogs a year are diagnosed with cancer with diet being the cause. Why?