It's the myth that all land must be somehow 'productive.'
Vast amounts of arable land are used to grow crops (soybean, alfalfa/lucerne, etc.) as animal feedstock, which isn't efficient land use of course. It'll be interesting to see the effects of climate change on crop growth, it's likely to force some significant re-evaluation of what we grow and where. We're very reliant on a small range of crops that have been extensively bred to return huge yields in carefully controlled conditions. Dairy cows might not be natural, but compared to something like wheat (an amalgam of three different species), most crops are freakish.
I'm mostly vegetarian these days, but I'm not sure I could give up my mild cheddar addiction. Trying to cut back on fish, but my wife is less keen on a vegetarian diet (mostly, I think, because she got electrocuted by a pig once).