Why can't we pass the path to a database other than RocksDB (e.g. ParityDB) to the Substrate Service?

Question

The Substrate Client has a:

• Substrate CLI here, where we're given the option to choose to use either RocksDB or ParityDB

• Substrate Service (sc-service) where here it states:

"The RocksDB feature activates the RocksDB database backend. If it is not activated, and you pass a path to a database, an error will be produced at runtime"

In its builder it sets the backend database (sc_client_api::backend::Backend) that "contains blocks and chain state" for a node here let backend = new_db_backend(config.db_config())?; using the sc-client-db database configuration, where we're also uses our choice of either RocksDB or ParityDB here.

In its tests here database: DatabaseSource::RocksDb { path: root.join("db"), cache_size: 128 }, and here source: DatabaseSource::RocksDb { path: tmp.path().into(), cache_size: 1024 }, it shows how it sets RocksDB as the database.

It also has a file with comment "RocksDB-based offchain workers local storage", where it appears to uses RocksDB here.

Is the reason why an error will be produced at runtime if we don't activate RocksDB with the feature (e.g. if we activate ParityDB and passed a path to ParityDB instead) because it only supports RocksDB for offchain workers local storage, or may we use ParityDB for offchain workers local storage instead?