For one thing, cats can't handle the level of salt in human diet. So to a human, cat food is very bland. This is also why one shouldn't feed cat salty human food.
It's part of the process of canning. The cans must be heated to kill all bacteria inside, otherwise anaerobic bacteria can reproduce in the can and cause nasty diseases like botulism.
If you buy a dinged or damaged can, you can often get a discount on it if you ask. They may or may not be safe to eat, and it's hard to tell until you open it.