It's survival of the fittest in the world of microorganisms.
"Superbugs," caused by antibiotic resistant bacteria, are becoming more and more prevalent in our world of modern medicine. A large reason for this is the excessive amount of antibiotics that are used on livestock to prevent diseases. To put things into perspective, an article from The Guardian notes that, "A whopping 70% of all medically-important antibiotics are sold in the US for use in livestock." With such a high volume of antibiotics being used, bacteria that are resistant are bound to persist.
According to that same article, many chicken producers have begun to decrease their use of medically important antibiotics within the last decade. The article assesses why chicken producers seem to be ahead of the game when it comes to reducing their use of antibiotics, and why it is important for producers of beef, pork, and other meats to catch up.