Plants require less land than animals. Shipping is quite cheap. Not to mention that lots of meat is shipped from developing countries to the West.
You cant eat the rainforest. Poor nations cant easily import all their food needs. Their population is booming. You are wrong to think k they only graze with cattle.
They burn the rainforest to grow plants already. They don't use it mostly for cattle. It is mostly used for sugar cane. Other crops as well, and obviously some grazing land.