Well, self reproducing to a point. Soybeans are self reproducing alright, but second generation corn has very different genetics that whatever was planted in the first place. It probably grows way worse too.
Now, the interesting question is why, specifically, we would consider that the GMO is riskier than a wild conventional crop. Mutations happen. Agrobacteria can alter the DNA of a plant in nature, just like it's done in a lab. We are just assuming that one is potentially dangerous, and the other is not.
Now, if what we wanted was a clear genetic lineage in the plants we eat, with well controlled genetics, agribusiness would actually love it, because big corn sells 100% identical hybrids. After a certain amount of testing, they'd be pretty darned sure that you are getting the exact same genetics you got last year, barring natural mutation that happens in any crossing. If I want to do the same through natural means, I need to either use the exact same hybridization techniques that you'll get from Monsanto or Dupont, or only grow inbred corn, which has terrible yields. No more replanting random seedcorn, because at that points, who knows where the pollen came from.