I think it's mostly down to the fact that the configuration space for database tuning is very large and that it can vary a lot from machine to machine. There's

I think it's mostly down to the fact that the configuration space for database tuning is very large and that it can vary a lot from machine to machine. There's a point up to which this would be better handled with a rules system. And after that "clever" stuff can search for rules. In any case the real problem with using ML is getting enough data. Most folks don't feel like spending millions of dollars waiting for their database autotuner to climb billions of hills, they'd rather someone else did it for them. Which brings us back to prebaked rules systems.