"Fat, drunk and stupid is no way for you to go through life"- Dean Wormer
"And you haven't answered my question. Why, if patents are property, do patents expire?"
Because patents aren't charters.
Those with knowledge of the history of patents, (quite evidently "not you"), know that in medieval Europe where the concepts developed, that there was a distinction between "charters" which were perennial rights and "patents" which were rights of finite duration.
Since we have finite rights to our inventions we deal in patents and not charters. If we had an office granting perpetual rights to inventions then it would be "The US Charter and Trademark Office" rather than the USPTO.
The patent term of 20 years, like patents themselves,derives from medieval practice and mirrors the expected working career of a craftsman in the medieval guilds. The first recorded modern patent for an invention was a barge fitted with a hoist for moving marble slabs. Florence, 1421.
LOL. You just made all that up. "Patent" is a medieval French word meaning open letter. They were most commonly used as royal declarations of titles of nobility. And no, they were most certainly not limited in duration.
And your attempt at invented history does not answer the question anyway. How can you call a patent a property right when it is limited in duration? Just saying it's because somebody else did it begs the question.