That's fair but that also suggests most actions hit the main database directly instead of caching layers. Possible, but somewhat unusual at this scale.
In quorum systems like CockroachDB, non-leaders provide tons of extra capacity for eventually consistent reads. [edit: maybe a bit less so in a big database because at any instant one machine should be a leader for some shards and non-leader replica for others.] It's not always worth the complexity of having a high-hit-rate cache in front of that. Maybe no cache is needed, or just one to mitigate the worst of the hot spots.