The reason is that Alibaba is stuck with mysql and complex queries being put very very deep inside their architecture.
And yes, things like top level transaction databases are like hand maintained clusters of 20 servers for the whole Alibaba with query caching servers in front of them to prevent OOMs
They tried to get rid of it many times as written in their tech blogs, but this probably indicates that they finally gave up on such endeavour, choosing to own Mysql instead
Their most sisyphean issue is the transaction locking. Imagine, 20 people trying to buy 19 trinkets, and you try to resolve transactions in a distributed DB, and two guys manage to buy the same thing in the exactly same nanosecond. The task is hard to tackle.
If you time-atomize the transaction, you get increase in minimal latency.
If you simply use a single server with very fast DB, you get reliability concerns.
And yes, things like top level transaction databases are like hand maintained clusters of 20 servers for the whole Alibaba with query caching servers in front of them to prevent OOMs
They tried to get rid of it many times as written in their tech blogs, but this probably indicates that they finally gave up on such endeavour, choosing to own Mysql instead