Game engines are one of the most inspiring and challenging pieces of technology a computer science can work on. On the one hand, they are so technically complex, touching so many diverse and motivating topics (from rendering, to IA, to resource management, etc.). Also they have to do everything fast, in an efficient way squeezing the most out of every platform. On the other hand, the outcome (or at least what they are used for) is art, something very visual, that can be touched and felt. So for a technical yet creative person like me, they are perfect.