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.
The thing is that when you have a huge non-citizen percentage of the population that is actively drawing taxpayer money out of the state to the point where the social welfare system is beginning to break down, and you have the working citizens of the country being taxed at 50% or more to support that during an escalating global cost of living crisis, you have effectively destroyed the social contract around citizenship that permits this system to function. For the massive aged population that's drawing retiree benefits, there's at least the justification that they paid into the system during their lifetime, even if the equation that makes the retirement system work increasingly doesn't work anymore.
Now the young people are being told to go die to keep that system alive. I wouldn't be surprised if most don't.
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.
More British Muslims joined ISIS than the British military. <https://www.nytimes.com/2015/04/19/magazine/her-majestys-jih...>