What's the source for it being easy? It seems easy, from the perspective of a human looking at results, but I'm not sure how much that's worth given the scale and complexity of the problem.
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.