There are two reasons why many companies run a closed source database. For some types of databases, there is no good equivalent in open source. For some types of workloads, the closed source implementations are orders of magnitude more efficient, faster, or scalable than anything in open source, so open source cannot reasonably support the workload.
It happens more often than you'd think. There are still many things in the database world that open source does relatively poorly compared to alternatives.
It happens more often than you'd think. There are still many things in the database world that open source does relatively poorly compared to alternatives.