Because we'd need significantly more land/pasture to feed livestock that way.
maybe that's just wishful thinking though.
If the cost of meat were a driving factor in reducing consumption, consider that it's maybe 4x less expensive (a guesstimate based on my own experience) to eat whole plant based proteins than meat.
It is inherently more efficient and sustainable to eat the plant proteins we grow than to feed them to animals. 1kg of meat also requires 100x the amount of water per kg of protein [1].