He could do that, but dual citizenship is hard to acquire in Germany so that might be a reason to not do it I guess.
Dual citizenship isn't a matter of "liking" but of treaties between the two countries. Some countries have it, some don't.
> special rules for EU citizens: They are not required to give up their previous citizenship in order to become naturalized German citizens, if their country of origin does not require Germans to give up their citizenship to become naturalized citizens of that country. [0]
[0] https://www.bmi.bund.de/SharedDocs/faqs/EN/themen/migration/...