I'm not sure the parallel worlds sort of created around Donna actually 'existed' in the way that an ongoing parallel world does. Once she died, she reverted back to the regular world, which obviously doesn't happen when ordinary people who cross into a parallel universe somehow die there. It's all very confusing; maybe to do with the way the beetle was involved in creating the alternate reality? Or maybe alternate reality != parallel universe?

I didn't like Turn Left, so I admit I only watched it once, but I assumed the parallel world where Donna and Rose were was a "fake" one, not a real one, manufactured by the beetle that was sucking energy or something.

I thought they said that reality bent around Donna and because she was so important, the whole of everything shifted around her because of the beetle, unlike what might happen with some insignificant bloke. BUT I should rewatch to see what Rose says about what she was finding in the different times/places where she was looking for the Doctor via the Dimension Canon because now it's all muddled in my head. Damn timey wimey wibbly wobble stuff.

I always assumed it was some kind of "pocket universe." They use that a lot in sci-fi to create little unstable temporary alt-universes that can be easily reversed to set the timeline right.