In many RDBMS this is in fact the case. Some take it up a notch, for example in postgresql even types are represented as a relation in the system catalog. Howev

In many RDBMS this is in fact the case. Some take it up a notch, for example in postgresql even types are represented as a relation in the system catalog. However, a system schema is necessarily very close to the database’s architecture and capabilities, making it implementation specific; commonality of underlying structure doesn’t really emerge from relational algebra because it is a) primarily a query paradigm and b) SQL is only a very loose adherent. In practice, woe betide the DBA that writes “insert into pg_enum” or “insert into pg_database” without knowing all the extra stuff that CREATE <anything> actually does. The DDL is also implementation specific but there’s a much higher degree of practical commonality. When writing application db migrations, and especially when supporting multiple backend databases, I’d definitely rather compose/generate DDL than engage with the minutiae of each.