We have way better food than they can get in the unprotected crop fields. We have food that tastes better, food that's easier to transport, food that can be stored for longer, etc. We have giant fridges and freezers and ovens to make cold food and hot food available without caring if it's cold or hot outside.

If we bred pigeons or rats smart enough, they could do all this stuff too, but they'd still be small. Their small size makes it easier for them to do other stuff that we will give them a lot of bread for, because it's easier for us to bake bread at our size. Pigeon clears leaves from a gutter for an hour, pigeon gets enough bread to feed 20 pigeons for the day, pigeon shares bread with his 20 pigeon coworkers and then takes a 20 day weekend while they do their shifts each day.