Diseases have developed over the years from pollution and crap that our immune systems can't deal with it. Technically, the human race is pampered, but lions and tigers and bears are not (unless they're in the zoo) so they still have to adapt to weather, new diseases, viruses, and bacteria to survive. If they had been eating cooked food since they were cubs, then it would be nearly impossible for them to adapt to new bacteria on raw meat if released into the wild. The other part of this is that most lions and tigers are inexposed to human beings so diseases transmitted by us won't reach their prey, either and it's no big deal eating raw meat. Remember the wild child from way way way back? He was probably able to eat raw meat. 1. because it was in older times which means less bacteria and disease and 2. he had been in the wild (they expect) since he was a baby. His body would've had to adapt to his suroundings.
The other part of this is that animals have better imune systems because it's in their nature to be in the wild. Along with adaptation and a very good digestive system, these three factors combine to make it almost impossible to get a bacterial infection from raw meat.
We, as human beings, are not as easily adaptable, we don't have great immune systems (flu shots, I believe are responsible. and washing our hands too much) and our digestive systems can only handle properly cooked meals (except for fish.)
This is the best answer I can come up with. Not saying it's right, but it's a theory I've had for a while and it's been researched. It's very propable so keep in mind my opinion. I'm sure you can google it and find the answer, too. ^.^
RESEARCH!!!!! ENDLESS HOURSSSS!!!!