They are engineered that way so that the newest generation of GMOs cannot interbreed with wild stocks--in other words, because of a concession to fear-mongers who predicted dire consequences if some GMO should start reproducing in the wild.
As someone else already pointed out, every seed company protects its intellectual and financial investment through the use of hybrid seeds.
I should mention that every food grown on a farm or ranch is, in fact, GMO. A number of methods for genetically modifying organisms have been developed over the last few thousand years. The only difference is that using modern methods allows us to target the modifications. So, instead of haphazardly crossing or radiating organisms causing random and unpredictable effects on the genome, we can target one single gene and change that one gene without touching the tens of thousands of other genes in that organism. If changing the genome of an organism is potentially hazardous, then modern methods have introduced a level of safety that was never before possible.
The danger lies in a patent owner seizing crops and/or suing a farmer out of existence.
But you probably don't believe there are people out there who want to control the world. What's the best way to control the world? Control the food supply.