And if all the old designs are still legal tender, why wouldn't counterfeiters just copy those instead? Artificially "aging" new copies of old designs is simple.
That's intentional. It's a psychological thing to keep the greenback accepted as the de facto "standard currency" in the rest of the world. Dull = staid = dependable = solid = the US dollar. You're never going to see garish, blinding, color-cause-it's-pretty, ultra-modern US currency that looks like this: