Because urgh, "high concept". Yes, PD is horror, which is one of the most looked-down-upon genres there is. It's also one of the most clever genres. Yes, it has its stinkers, but so does every genre. Anyway.
PD's writers aren't just clever for their (fucking blatantly obvious) Shakespeare references. There's a lot of stuff hidden in there once you look past the "gory amazing ridiculousness". There's the question of how far are you willing to go to achieve your goals. There's the question of responsibility for your actions. There's the whole theme of redemption. There's the question of where does humanity begin, and where does it end.
And the literary references for sure don't end with Shakespeare. I'm wondering if that person has ever read "Frankenstein". That book is considered a classic not just for being one of the foundations of a whole genre, but also because it touches on some very deep and important themes on what it means to be human, the fear of what is different and how revenge is more destructive than anything else. And PD follows that course with its interpretation of Frankenstein. Then we have Dorian Gray, with its themes of hedonism, dupicity and narcissism. I'm pretty damn sure that Caliban is also a reference to "The Phantom of the Opera".
Just because something is shocking and bloody doesn't mean it's not deep. I thought we all knew that after Hannibal hit the screens.