Lag is your answer. We've had times in the past where there's been too huge of animal farms, that the server lags, and eventually crashes. Sorry that we cannot implement it, because believe me, i'd like it if there was breeding too.

 

There is a limit to farm sizes in place though. So why not let this happen? Would make the meat market flourish.

 

There was a plugin I once saw on another server where if a certain area ( I think 20 by 20) had more than a certain # of entities the server would butcher some to keep it to the limit. It didn't work though since the plugin itself added a lot of lag and it just became a messy server with people coming up with ideas to make the plugin not work.

 

If the number of entities per block could be limited, wouldn't that make aggressive mobs viable again?