You _can_, but pasture chicken and pigs aren’t cheap and aren’t really done at a commercial scale. Whereas many conventional beef farmers still raise cows on pasture.

Overall, pasture chicken requires a lot of land and has soil loading issues (too many and the soil gets burned out when the balance of minerals goes off kilter). It’s much harder to overload cows on pasture - it’s obvious when a field is overgrazed.

Pasture pigs are labour intensive, too.