One reason it might deteriorated is that goog is constantly battling ppl 'optimizing' their content for Google while competitors likely see less than 1/1000 of this.
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.