> While I agree with you, I'm struggling to find a good argument why intellectual property should be treated differently from physical property

1) First reason, it is not physical property.

2) Second, many of the creators who lobbied for longer copyright terms benefited from a rich public domain when they did not have anything. In the case of Walt Disney, he made a film series based on Alice in Wonderland, which never had copyright protection in the US, and it's copyright expired in the UK in 1907.

https://en.wikipedia.org/wiki/Walt_Disney#Early_career:_1920...