That's simply because you haven't looked. There was literally a black hat talk on exploiting cellular networks this year. Every year there seems like there's a new exploit. Just because it doesn't make it to your particular side of the internet doesn't mean it's not happening.
Now, can anyone definitively prove that they are back doors and not mistakes or exploits? No. But that's thanks to the age old "Never ascribe to malice that which is adequately explained by incompetence."
Now, can anyone definitively prove that they are back doors and not mistakes or exploits? No. But that's thanks to the age old "Never ascribe to malice that which is adequately explained by incompetence."