When you consider arability constraints and nutritional efficiency, meat is more efficient and less destructive than plants in many cases. Soy production, for example, sufficient to meet global demand requires destroying the Amazon; meat production is much less destructive to land, taking place either on non-arable scrubland or via consuming human-inedible surplus feedstock. (Or on land already depleted by plant farming, like in the Amazon.)
And then in the end you're left with foods with inferior nutrition. I'm not going to stop eating the foods I've evolved to eat (ruminants especially) until I can safely do so without becoming malnourished, weak, and unhealthy. Right now, this is not technologically possible.