Comment by Yokohiii

RDBMS are typically pretty good keeping the frequently requested data in RAM. This disguises the latency of disk access and performance will heavily depend on access patterns. If you serve 1TB of data from a DB with 8GB of RAM and that is sufficient for your use cases, I wont stop you. If you expect low, predictable latency (<1ms) even on a 98/2 r/w system, then it it's not worth the headache.

Of course everything depends on use case and constraints. I highlight the extremes here, the initial confusion was why DBs require so much RAM. Traditional DBs are optimized around RAM, that's where they perform best. You can abuse that, but it's not the best they can be in terms of latency, predictability and stability.