Look at how fanatic the compatibility actually is. Building Postgres or MySQL is conceivable but probably will require some changes. (SQLite compiles and runs with zero changes right now.)
It does (or did, at some point) pass the thorough SQLite test suite, so at least it's probably correct! The famous SQLite test coverage and general proven quality might make SQLite itself less interesting to harden, but in order to run less comprehensively verified software that links with SQLite, we have to build SQLite with Fil-C too.