I'd like to think that seeing as though Megatron is missing half of his brain and what is left is clearly invested with cyber-parasites, that he is in a very delusive and detached mental state - very akin to a borderline psychotic vegetable-like state.

Hence, he just sits there in a world of his own as he watches his beloved Cybertron appear before his optics, grinning with a deluded delight... cue Carly.

