Default legal rules can change over time and place. The general rule is that contracts are strict liability obligations and it isn't always clear what circumstances suffice to constitute an exception to that general rule.
Explicitly including a definition and exception avoid the risk that the local default rule will not be the one intended and also provides a way to prove the exception to a non-lawyer without resort to reference to case law or statutes.
Lawyers say "including, but not limited to . . ." to avoid the inference that "including . . ." sets forth a complete definition of the events that are covered.
Again, the default definition could be used, but can vary over time and place and might leave more room for dispute over the meaning as many legal terms have different meanings in different contexts and as the case law definition terms is not always clear or may not even exist in every jurisdiction.