Because citizenship is a commitment, not merely a set of privileges. If you obtain a citizenship (or have it imposed on you upon birth), it comes with a set of duties other residents and visitors do not have. The duty to defend your country is a traditional one.

Some countries may conscript non-citizens or allow them to serve voluntarily. Often because they are more likely to use the military as an extension of foreign policy rather than for defense. Others may see it a waste of effort, as those people are probably not sufficiently committed to the continued existence of the country.

Because not only that the immigrants have no allegiance to Germany, most have different culture and sometimes incompatible values.

I grew up in the Middle East and I can tell that cultural differences and values were more smooth and compatible than what I saw in Germany. Conscription requires a degree of trust in the people you give guns to and expect to fight on your side in case it is needed, that is mostly not true with immigrants in all times and all countries.

Because immigrants have the right to be protected, while native men have a duty to die for their rulers.

Even if they are included, they wouldn't join up.

More British Muslims joined ISIS than the British military. <https://www.nytimes.com/2015/04/19/magazine/her-majestys-jih...>