User blog comment:Omega natsu2/Chapter 397: Steel, Predictions./@comment-25111584-20140816114437/@comment-4829046-20140816163941

Plus they are minor characters not major antagonists at the time. Only one of the Monkey brothers(Zato) was even prominent as a disruptive entity during peaceful times and that was only in the manga since the anime delayed his appearance till later, possibly due to censorship issues. Nevertheless, he wasn't a threat to a FT mage.