Because growing 1000 calories worth of a plant takes less space then growing 10000 calories. And if you want a 1000 calories of cow meat you will need to feed it much more then 10000 calories of plant.
For instance, New Zealand lamb flocks are grown high up in the mountains, eating and drinking from local sources unsuitable for plant farming. It would literally take more arable land for the equivalent plant based diet.
That's such an edge case though.. Burger King and Mcdonalds aren't selling lamb burgers in large quantities.
What about all the cattle farmed all over the world? They're not all on a steep mountainside. Most will be on plains or more factory like conditions.
What is a generalization and is not true in many cases is that the meat we eat is produced in environments unsuitable for plant farming.
Maybe not oversimplification but deceiving. It makes it sound like we can eat grass.
When you're talking about scale, feeding 10's of thousands of people, plant based food sources win out.
Without those animals, that land would have been literally useless for humans to produce food.
Lambs are usually slaughtered after 6 months. In Sweden, to use an example close to me, lambs have the right to live outdoor for two to four months during the summer depending on where in the country they live, which means that they spend at least 2-4 months indoors not grazing this non-arable land. They must be watered and fed, which increases energy consumption, and usually fed from land that could have grown crops for humans directly. Only a fraction of the energy we put into animals gets returned when we eat them, which means it's far more energy-efficient to eat the plants directly.
That's not necessarily true. I think you are underestimating the amount of land that's suitable for animal farming but not crops. It's a lot.
> Only a fraction of the energy we put into animals gets returned when we eat them, which means it's far more energy-efficient to eat the plants directly.
That's technically correct but still wrong. We can't eat grass and lichens, no matter how many calories they have. Animals can.
Other resources, such as fossil fuels and water, are used at a much higher rate when raising animals than when producing crops. Grain-fed beef production uses 100 000 liters of water per kilogram of food produced, broiler chicken 3500 liters per kilogram, and soybeans 2000 liters per kilogram. Water is a precious resource in many places of the world.
This is the way the world works today, but you don't argue for animal products over vegetables, you argue for grass-feed animals instead of crop-fed. You should vote with your wallet and only buy grass-fed meat, both when you cook at home, when you buy an animal-based snack, or when you go to a restaurant. I think that many meals would have to be vegetarian, because a purely grass-fed meat, dairy, and egg diet would be hard to uphold.
Weather those calories were going to be used by humans or not that land is now used to feed some cows that will feed a few people. Instead we could use that land to grow plants and feed more people then with cows.
Also, AFAIK, that natural environment where cows are fed is mostly created by destroying forests.
At least in the United States, there is a fair bit of land that is both non-arable and yet perfectly sustainable for cattle (much of this land was historically home of a bovine species, the plains bison, in the first place). I'm thinking areas like certain areas of Texas, Wyoming, Montana, etc.
There are probably other areas in this regard, so I think it's possible to keep some meat and actually have it be relatively sustainable.
Everyone's overall point is correct, however, in that this does not apply to a lot of meat production today; keeping only the more sustainable practices would end up making meat a luxury product (as it was in earlier times).
Sometimes we can't grow crops on that land. And it's a lot of non-arable land.
That space for grass feeding could be used to grow much more food to feed humans directly.