Maybe it's because while an individual Transformer may not be as challenging as a Rubik's Cube ('cept MP Megatron ), Transformers offer a greater variety and diversity of challenges as a "puzzle toy." Also, the Rubik's Cube is just a puzzle toy, whereas a Transformer is a puzzle and an action figure and a toy vehicle (or whatever other alt mode). So it's like three toys in one (or more if they have multiple alt modes). And then you have the creative/imaginative aspect of the mythos surrounding the franchise; not only is there the background story of alien robots at war, but each robot is an individual character each with their own unique personality traits, abilities, weaknesses etc.