Short answer: because while copyright can secure and bolster freedom of expression in the short-term, nearly infinite copyright extensions stifle creativity and expression becomes prohibitively expensive.
If an 11-note sequence counts as infringement, how much do modern artists owe Pachelbel’s descendants? The four-chord sequence making up the core of his Canon in D has been repeated in dozens, if not hundreds, of subsequent songs. Should evidence produced by Australia’s Axis of Awesome be used in copyright lawsuits by anyone who can document that, ten generations back, Johann Pachelbel was a great-great-grandfather? It seems absurd.