Low pay, high expenses. Most rehabbers are non-profit organizations and are often focusing on fund raising. Breaking into this field is simplest by contacting the local wildlife rehabber in your area and doing volunteer work. These positions are generally in fields such as parks that hold animal shows, circuses, and filmwork. The training required to get there primarily is experience .
Most trainers appreciate teaching their employees with their own methods.
Therefore, it requires the least amount of schooling. However, it also requires much time spent doing work for little or no pay in the beginning years. And counties have Animal Control offices. All of these dept's employ biologists, wardens, veterinarians, etc. Generally, they will all require a college degree.