There probably isn't an actual reason other than "nice round number," but this could conceivably be something like a typical time to expect 50% of a corporate o

There probably isn't an actual reason other than "nice round number," but this could conceivably be something like a typical time to expect 50% of a corporate organization to have turned over. 100 years would be guaranteed 100% turnover, including ownership, and likely 99% of organizations don't even exist that long. At that point, it may as well just never automatically expire.