Because a reasonably well optimized database with support for indexes, data integrity enforcement, transactions, and all the other important things we expect from a good (relational) database is complex enough that it takes a rather large codebase to do it reasonably well. It’s not something you slap together out of a handful of function calls.

ETA: look at SQLite for an example — it’s a relatively recent and simple entrant in the field and the closest you’ll find in the mainstream to a purely filesystem based RDBMS. How would you provide a stdlib that would let you implement something like that reasonably simply? What would be the use case for it?