Because land-use change is a leading cause of climate change, and most land-use change is happening to grow and harvest animals. Add on top of that the monoculture crops are also largely grown to feed animals, and those monocultures also drive climate change.

Eating a plant based diet is great, but even switching to locally raised, pasture grown animals can be a positive. But ultimately pastures need to be re-wilded into natural forests. Beef needs to become an expensive luxury and not a dietary staple.