A theory proposed, and that i agree with, by a nuzlocke player is that reshiram and zekrom weren't just created from nothing as that's not really how energy and matter work for the most part, although there has been cases where energy has been used to make matter its just easier to bestow more energy upon something than use it to make something. Essentially it goes that before the unova war the two princes went to kyurem and asked for its help and power to pursue truth/ideals. the dragon hearing their pleas bestowed its power upon them, however where the legend gets it wrong is that the dragons weren't created from nothing but are the princes themselves transformed after having the energy of the dragon put into them.