First off, a mud typically needs signficant parts of the world loaded at any given time. As such, you would need to implement a "hey, clear the cache" mechanism to force a content reload, which looks suspiciously like a copyover. Secondly, it makes it more difficult to exchange content between instances of UncleverOMG! unless a file format is created for that exchange. As it's just as easy to load from the filesystem periodically as it is from a SQL DBMS, this makes the SQL DBMS unnecessary.
Using plain files for content storage makes the exchange of content very easy. Additionally, since quite a few muds are hosted on Linodes, other virtual machines, or even shell providers specializing in mud hosting, a SQL DBMS isn't always available or desirable. It also adds a layer of complexity that isn't necessary.
Arguing about VNUMs is pointless. VNUMs aren't needed, ever. All that's needed is a unique identifier. We get that by the nature of using a pseduo-fs naming convention.