We do if we want everyone to eat. Grazing animals are a great way to turn stuff we can't eat into stuff we can eat, and stuff we can use to grow crops that we can also eat.
The vast majority of ruminants are fed corn and soy. If they were all to eat grass we'd be able to produce much less meat than we currently do, and also use much more land
Are you trolling or is this mood affiliation? Have you even really looked into the impact of the food miles of the ingredients you're using relatively to the rest of their carbon footprint? Spoiler: your food miles aren't going to make much of a difference relatively to the rest. Obviously, you can imagine a strawman vegan eating only your proverbial plane-flown avocado and compare that to a perfect locavore eating only grass-fed meat, and sparsely so. But how about compare the real average "locavore" and "vegan" and seeing how that goes. The BBC published a nice piece [1] (n=3) that gives a few pointers to help audit the impact of your food miles. Mileage will vary but even in the UK, with its heavy reliance on imported food, food miles weren't that big a deal compared with opting to eat less meat.
This is not true. There is plenty of food, especially if we put all resources we currently feed livestock into feeding humans.