First thing I thought is that humans must be paid for their work, which is not something that O.W.C.A. wants. But then I realized that Carl is human and he is not paid for his work. Don't forget that Carl is actually very useful!
So, why not recruit more humans that are very useful and not need to be paid just like Carl? You might say that not many humans would want to work for free, but there are definitely people like Carl who would. Is it because non-human agents are better suited for their tasks? If so, then in what way?
I even remember an episode where flower was recruited as agent and flowers are not even animals.