One of the greatest difficulties in wildlife rehabilitation is not being able to support every animal. Sometimes, despite doctors' and rehabbers' real efforts, animals are not able to be discharged. In order to survive in the wildlife and be releasable, animals require to be 100% healthy. If a bird could not fly, or a mammal could not see, wildlife rehabilitators could not free them back to the wild because they will not sustain.
Wild mammals don't have masters that pay the bills for their treatment. Most wildlife rehabilitators and dispensaries depend on charity from human sponsors to fund for the food, medications, and other supplies they require to care for wild animals. Finding the money required to treat and rehabilitate wildlife can be very challenging.
Injured or ill creatures in custody can be a menace to those attempting to help them. Birds of prey like falcons and owls have powerful feet with sharp hooks that can harm people. Many creatures like raccoons and even squirrels can nibble very hard.
