That's because they were never going for retro styling with the new Chargers. They did however take styling cues from the old ones. 06-10 took cues from the 68; circle taillights, the recessed rear window, most of the curves of the fenders, the ducktail on the back end. Even the shifter gate resembles the one in my old 72 (and that design goes back to 68). 11-14 they took cues from the 69-70's. Bar taillights, clamshells on the doors and hood depressions. Even the stock spoilers were made to resemble the old Go-wings.
Now the Challenger? They openly admitted that they rolled a 1970 into the design studio when they were designing the 2008. For 2015+ they moved up to more of the 1971 styling cues.