It's because they are close together in the factory farm. I don't think you need to consult what veterinarians say - seeing photos of how animals are treated should be enough for you to just stop consuming meat (if you "think about the wellbeing of the animals").
Consider watching Earthlings (2005) - http://www.nationearth.com/ - I'd say a must watch film for anyone who cares about animals.
Ranches surrounding me (I live in relatively rural Montana) don't even come close to resembling the kinds you mention. The cattle are given the run of hundreds of acres, and also they often graze from those fields, etc.
Yeah, there are a shitty minority of ranches that produce a large amount of meat in terrible conditions. But they are not the norm, not in my experience.
And for a tangent, I'd like also call out that it's not just beef being produced by ranchers. We're not just tossing carcasses in the landfills. We use the whole animal. Calcium, leather, feed, gelatin, medicine, etc.
You are going by your personal experience which is very dangerous. As far as I understand statistics (taken across the US), If I remember right, at least 97% of all meat comes from factory farms (depends on animal, this may not be aggregate across all animal types).
So, most people want to believe their meat comes from somewhere nice, but on average, basically all meat in the US comes from animals that are living in horrible conditions (I suspect living lives not worth living -- a life of suffering).
"We estimate that 99% of US farmed animals are living in factory farms at present. By species, we estimate that 70.4% of cows, 98.3% of pigs, 99.8% of turkeys, 98.2% of chickens raised for eggs, and over 99.9% of chickens raised for meat are living in factory farms."
Being honest though, it's sus as fuck, and not just because of the source, or that these are "rough estimates" to use their terms.
A simple read through the spreadsheet shows some pretty odd (and significant) discrepancies. A single example: A row with "2500-4999" animals per farm has farm counts and "total animals" that amounts to over 6.5k animals per farm.
Also, note that CAFO - the farms we're (legitimately) concerned about - is not based solely on the animal counts†, though that's the only part of the definition that the "Sentience Institute" uses because "the public may consider it bad too".
It strikes me as straight up lying with numbers - presenting real numbers in a way which tells the story the institute wants to tell.
† "has a manmade ditch or pipe that carries manure or wastewater to surface water; or the animals come into contact with surface water that passes through the area where they’re confined."
This is why I super appreciate and love the book Animal Liberation (1975) by Peter Singer -- a classic that started modern-day vegetarianism.
The author, my favorite philosopher, uses industry booklets and instruction manuals as examples of what happens at the farms (and you know worse things happen than what is described). It's horrific stuff, enough to make the reader want to decrease their meat consumption. I'm 99% sure that since its publication, the % of animals coming from CAFOs has increased. And since then various other problems appeared (chickens genetically engineered to grow so fast that often their bones break -- resulting in more suffering than before).
Consider watching Earthlings (2005) - http://www.nationearth.com/ - I'd say a must watch film for anyone who cares about animals.