Why do Muslims join kuffar armies?

And who said anything about finding a law or religion stopping Muslims living in the West?
This is a simple question. If “kuffar enemies of Islam” are waging war against Islam then why are you guys living among those kuffars? Aren’t your taxes helping them in their “crusade against Islam”?
And it is not just about monetary benefits you are providing to “enemies of Allah”. By working in such countries, you are also providing them intellectual benefits.
Why gives those benefits to “kuffar, enemies of Islam” instead of “mujahideen, Fazlullah and Mullah Omar”?