It's not that you have to, but you have a choice and SQL Server is as valid as any other choice. It isn't about picking one OSS component and then being limited to only other OSS items.
You forgot license costs, which is easy for many of the folks here. If you already have enterprise level agreements or a large budget you may be cavalier with adding yet another SQL Server instance or server. But if you have OSS expertise and are cost sensitive, you may not need to spend the additional $$$$$ for a RBMS or NoSQL system.
(I live in both worlds and have a wonderful time explaining to clients why the license costs of SQL Server are 3 to 4 times the cost of their state of the art hardware.)