That would make sense if so many easy to detect low hanging fruit like blog spam, scraped stack overflow pirates, and listicles didn't make it to the top. Those are easy for google to de-rank and yet they don't.
But the issue isn't that they can't; the issue is that they don't want that. Why the sites with copied content exists? To earn money through ads. What earns Google money? Ads!
And any simple heuristic is quickly reverse engineered by SEOs, who will find a way to mask it as legitimate.
tl;dr it's a hard problem.
As I have said, the reason they don't do it is not because they don't have the skills and know-how.