I think part of this is that most of the common knowledge about scaling is hard fought from the 90s/2000 era. eBay got bigger and bigger Sun boxes to run Oracle, until they couldn't get anything bigger -- then they had a problem and had to shard their listings into categories, etc. In the last few Intel cpu generations, computation performance has had small gains, but addressable memory has doubled about every other release, you can now get machines with 2TB of ram, and it's not that expensive.
You can fit a lot of data in an in memory database of 2TB.
You can fit a lot of data in an in memory database of 2TB.