However, to answer your question, the reason is right in the linked article… the first order benefit of allowing everyone to copy everything freely makes the world a better place… if everyone can download and read a great book for free, everyone gets a free book and no extra resources are used… that is a lot of utility created.
Of course, the worry is the that the second order effect will be fewer people create works because they can’t support themselves if everyone can just copy their work.
So we create copyright to mitigate that second order effect, but we still want to maximize the first order benefit of as many people getting access as possible… so the best way to do that is to set the copyright term as short as possible so that people are incentivized to create as much work as possible, but still get the benefit of free copying as soon as we can.
As the author points out, no one is going to decide not to create a work just because they know they won’t be able to profit from it in 70 years… if people are still going to want it in 70 years, that means it will be very popular and certainly financially still worth creating even if you can only profit for say, 20 years.
So the tl;dr… we want copyrights to expire because we aren’t incentivizing more creation by having copyrights not expire. Basically, the ideal would be to tune the length to keep being shorter until we see a drop off in output of creative works.