It's a file that contains data. What is a database if not that?

It's installed as part of an instance, and used by SQL Server. What is a system database if not that?

Microsoft (the people who designed the product) state unequivocally that they regard it as a system database. Who has the right to decide on its classification if not them?

It's not like other SQL databases. So what? How does that make it less of a database so long as the above criteria have been met?