You're not talking about animals in general though. You're talking about cows. Because every other animal we eat, eats something we can't eat. Sheep and goats eat hay, pigs and chicken eat garbage basically and fish eat ... ocean garbage.
And while it's trut that we can't feed 10 billion people on cows, that's also true for the fewer billions we have today: the most commonly consumed kinds of meat are chicken and pork, not beef. Beef is the most commonly consumed kind of meat in affluent western countries that are also home to the most vocal vegan and animal rights movements, which goes a long way to explain why many of the discussions about what we eat say "meat" when they mean "beef".
And while it's trut that we can't feed 10 billion people on cows, that's also true for the fewer billions we have today: the most commonly consumed kinds of meat are chicken and pork, not beef. Beef is the most commonly consumed kind of meat in affluent western countries that are also home to the most vocal vegan and animal rights movements, which goes a long way to explain why many of the discussions about what we eat say "meat" when they mean "beef".