That's actually a good point, even if sold as a separate drive. I suspect the blue laser module itself may be the issue, e.g. if they can't make the blue laser pick-up head thin enough, this would prevent them shrinking the overall thickness.
The main thing that amazed me several years ago was the Compact Flash type-II card, which was actually a spinning HDD the size of a compact flash card, but just a little thicker. Even today, I still wonder how they managed to pack the motor, spindle, platter, pick-up head, etc. in that small space.