My beef (no pun intended) is that crops and livestock are not spreadable. Horses were introduced to the western hemisphere and bred; now wild herds roam the west and can be rounded up and tamed. Corn was introduced to the eastern hemisphere, and is cultivated there today. Many other examples exist. And, after all, the central US isn't just a bunch of grass farms -- its corn, wheat, soy, and other important grains and produce.
Obviously climate plays a factor, but it would kinda be cool if a city with a certain resource (cow, pig, corn, wheat, et al) could create a "missionary" type of unit that could be used to spread a resource to another tile for a mild bonus. Give it a certain likelyhood of "taking", based on latitude, terrain type, and era of gameplay.
But alas, that would require a lot more programming and isn't really in the essence of the civ series. It probably wouldn't add much to gameplay except another level of pointless complexety. This is a game, people.
This concludes your RL check