Why did Steven Spielberg invent a new dinosaur for Jurassic World?

I am an unabashed Jurassic Park fan. I vividly remembering jumping out of the car and running into the theater when the first movie came out in 1993, and I vividly remember jumping into my car and driving away from the theater as fast as possible when the last movie came out in 2001.

So I watched today's newly-released Jurassic World trailer with great anticipation, and was left with only one question: why the hell did they invent a dinosaur? The plot of World turns on a new strain of "hybrid" dinosaur, which causes a befuddled Chris Pratt to run around with Very Serious Expressions — all of which might make for a highly entertaining movie, but not a Jurassic Park movie. The whole point of Jurassic Park is to bring real dinosaurs into the present day — that's why the T. rex and raptors in the first movie were so much fun, and why the awful second movie was redeemed by the T. rex smashing up San Diego. The third movie was terrible, but the Spinosaurus and Pterenodon were terrifying —because they once actually lived on earth. Take away the real dinosaurs, and you're just left with a made-up monster movie.

So why did Steven Spielberg and director Colin Trevorrow feel compelled to invent a new dino for this hopefully-triumphant return to the screen?

@reckless "we need clear IP on the dinosaur, the toy companies say they can’t have any copycats this time"

— Tim Carmody (@tcarmody) November 25, 2014

Oh.