First, it should be pointed out that humans are designed to be vegetarians. As evidence, look at our teeth. Our teeth are similar to cows (herbivores), and nothing like dogs (carnivores).
Second, eating meat is all well and good a thousand years ago when there was a small population of people. And it would be fine now, as long as there were only a small proportion eating meat. However, with a huge population eating meat, problems are caused.
Go read Hardin's "Tragedy of the Commons". He talks about exactly this problem. A few people raising cattle is fine, but with more people, there's more competition. With more competition, they will find cheaper ways to do things, such as growing more cattle on the same amount of land, feeding them low-grade feeds, and keeping them too close together. This encourages lower quality meat, environmental damage, and spread of disease.
The problem stems from profit. You make more money if you damage land more. And if some farmers do it, everybody else has to. Otherwise, they won't make as much money and they will go broke.
The unfortunate truth is that the Tragedy of the Commons is on top of us right now. Look up the essay, it's really good.
