The problem is not just the performances, it's that distributing has a huge cost in term of servers and maintenance.
If you can write only 50 times a second, your data set won't get big enough to justify distributing it.
Put your millions of row in one server and be done with it. Cheaper, faster, easier.
There is a tendancy nowaway to make things distributed for the sake of it.
Distribution is a constraint, not a feature.
Can you really not see why distributed databases are needed? High availability, (geo) replication, active/active, oversized data, concurrent users, and parallel queries are just a few of the reasons.
Distribution will always come with a cost, but the tradeoffs are for every application to make. We use a distributed SQL database and while it's faster than any single-node standard relational DB would be, speed isn't the reason we use it.