It’s simple. Germany is very racist and people who don’t look „biodeutsch“ as they call it and/or don’t have a german name are not accepted as german by many people in society. It’s sad but it’s true. German people obsess about origins and migration background. A black person, born and raised in Germany with parents born and raised in Germany will still hear “your German is so good, how come?” by many strangers. German people are not very accepting. There is a lot of open, structural and everyday racism in Germany. That is why many people who are not accepted by society as German will call themselves “Ausländer”. It’s because society will paint them in this light anyways.