It's not just about type safety, though. A good RDBMS enforce things like referential integrity (FOREIGN KEY constraints) and allow you to express further constraints on your data (e.g. order.amount must be a positive number, a certain combination of columns must be unique across the table, etc.)

And most (all?) RDBMS that have been around for a while have been tuned and optimized to support this efficiently. I remember reading that at some point (1980s-1990s-ish) DBMS vendors were buying compiler developers like crazy to help them with query optimization and such.