Another one because they make money.
There's a fallacy that they make sequels and remakes because Hollywood has run out of ideas. Frankly, ideas were already run out centuries ago in myths, legends, epic poems, fairy tales, theater... wouldn't be surprised if most tropes were considered overused before written word. Hollywood makes remakes and sequels because, for the most part they're low risk. If a movie does well the first time around, guess what seems like only a logical film to invest in? Remakes...eh... those are a little more risk on the count of if you do it the wrong way (and they often do) you alienate the fanbase of the original and fail to capture the audience that has never heard of it/doesn't give a crap. Chipmunks was successful. That's almost an exception to the rule. For every Chipmunks/Transformers (say what you will, they make tons of money)/Get Smart/Dirty Rotten Scoundrels (NO ONE remembers the not Steve Martin one), there's dozens of Marmadukes, Lone Rangers, Green Hornets, that remake of Psycho that no one had any business doing. Even if they don't make remakes, they wind up making similar films with slight twists (like how R.I.P.D. is essentially MIB with death).
I just have to ask... what "original" films do people want to actually see? Original usually translates into pseudo-independent movie about feelings and relationships... and lemme tell you, that stuff is just as far from original as a sequel to a remake of a remake of a sequel. Oscar Bait films are invariably old people in a relationship slowly dying, something crappy that happened in history (usually genocide), an emotionally manipulative Biopic, and something about how bittersweet everything is. Are they better films? Debatable. But they are no less unoriginal than yet another Chipmunks film.
I'd say original would be staring at a jar of mayo for 2 hours... if Rocko didn't already do it.