Animals can digest stuff we can’t. We can eat crops because we bred them for many thousands of years to be mostly edible by us. Early wheat, for example, is hardly different than grass. Animals are much more efficient at turning grass into energy than we are, and grass grows all over naturally without any effort. You don’t have to plant it, water it, or anything else.

Yes, meat is much less efficient than, for example, eating the corn ourselves. It isn’t less efficient than us eating grass ourselves. In that state, we basically can’t survive. There are some native plants we can eat still, but usually they’re seasonal and not overly abundant. You have to move around to survive, hence hunter-gatherers.