it's explained in the video, and there's no way I'll be explaining it better than her
The actual explanation starts a couple minutes later, around https://youtu.be/KKbgulTp3FE?t=1553. The short explanation is performance (essentially load balancing against multiple RAM banks for large sequential RAM accesses), combined with a security-via-obscurity layer of defense against rowhammer.
The three answers it found were:
- Avoiding lock-in to them: http://www.youtube.com/watch?v=KKbgulTp3FE&t=1914
- Competitive advantage: http://www.youtube.com/watch?v=KKbgulTp3FE&t=1852
- Perceived Lack of Use Case: http://www.youtube.com/watch?v=KKbgulTp3FE&t=1971
Those points do actually exist in the video, I checked. If there are more, I don't know about them, as I haven't yet watched the rest of the video.