One sick cow in an industrial place poisons millions of pounds of beef. One sick cow in a traditional place poisons hundreds of pounds of beef.
The math works like this: what are the odds of every single animal in the batch being healthy? Because nothing else matters, one sick animal contaminates the batch.
Your chance of getting a burger that won't make you sick is
(1-P)^N
where P is the chance of any one animal being sick, and N is the number of animals. Assume P is constant between the traditional and the industrial, and look at what happens with large N. Look at an industrial farm and a traditional farm and tell me P isn't way less on the traditional farm, which makes the numbers far worse.
The math makes me suspect there are a lot of cases slipping by, contaminations that aren't caught, disease outbreaks that involve lots of people feeling pretty grungy but not going to the doctor or dying, or not mentioning the burger they had a week ago. Because even with a very small value of P, as N gets big, your chance of a non-toxic lunch goes down. Way down. Plug in some numbers and play.
There are economics of scale, but when I buy a quarter of a cow from a traditional place, I pay as much per pound for the whole thing as I do for low-grade industrial burger. And there is nothing in that quarter cow that would cost less than the low-grade burger.