There are and have been plenty of people that have eaten petfood and no known (to me anyway) harm has come to them. But....
See raw product coming into the factory, the trailers / containers are not sanitised as they would be for human consumption, if they're sanitised at all apart from a jet wash perhaps. Where cleaning chemicals are used, they almost certainly won't meet regulations for human consumption.
Some buildings SOME products are manufactured in are wholly unclean. On the flip side, Petfoods former factory in Peterborough was probably cleaner than some human food factories ive worked in.
The machinery is not sanitised as would a machine processing products for our consumption...again, often just a jet wash. Ditto the chemicals used.
Mince chicken waste in those machines, its THE most God awful fatty mess that gets left behind, a devil of a job to clean, let alone properly for human consumption.
The handling of raw product during processing will nowhere near meet regulations for our consumption, often left out in various temperatures, open to flies, wasps etc etc.
There are very very strict regulations following the BSE thing regarding what parts of an animal can...and more importantly most definately cannot be used in human consumption products...i don't know if the same applies to petfood products...although i suspect the same rules apply, they may not be applied so vigorously by dubious suppliers down the chain.
Our factory was producing right at the dirty end of the process...thye stuff Petfoods didnt want to do, but simply farmed it out to maintain the cleanliness of their plant. Its a filthy process whoever does it unless you spend millions.
All that said, ive spent 40 years working in food processing plants....and conditions in human food plants occasionally make you shudder.