Sorry, I should clarify...it's not in every cat/ dog food. The ingredients to look out for are 'meat & bone meal', 'poultry meal/ digest', animal fat/ digest, etc...the unnamed, mystery meats. Those are rendered from the euthanized animals, 4D meats, etc.
There are hidden videos all over utube exposing what really goes into making pet food...there are rendering plants. Link below is how to get a Dead Animal Transport/ Dealer License.
MDARD - Dead Animal Dealer/Renderer License
All the foods that are unfit for human consumption goes into making cat/dog food (again, not all pet food). They're not neatly packaged up & transported in refrigerated trucks.
I would look for a pet food company that says they use organic USDA (grass fed meat, preferably), one that doesn't hide where their ingredients comes from. You can always contact them & ask.