[mjb]

Even more generally, distributed systems can find simpler solutions to things like "raise the throughput ceiling", and "handle disk failure", and "handle power failure" than single-box systems. This is for the simple reason that they have more options: beyond the constraints of a box, resource allocation is more flexible, failures less correlated, etc. That allows modern distributed databases to simply avoid some of the super hard problems that prior databases had to solve. Efficiency is still important, but the thing to optimize is mean system efficiency, not the peak performance of a handful of super hot boxes.

There's also the fact that decades of DB research have brought techniques and approaches that beat old ones, and retrofitting existing systems with them can be hard (e.g. see the efforts to remove some of the sharp edges of PG's MVCC behavior and how hard they've turned out to be).