> "no one would do this work voluntarily"

The much more common reason is compartmentalisation. Employees are told as much as they need to know, no more.

If someone can design a glide bomb without knowing that it has an explosive payload, then they're not told.

The fear is not so much the employees themselves (they might be quite patriotic!) but that the information will leak out to the enemy, giving them a chance to counter the weapon or copy it.

That's a very different proposition to what the various parent posters are implying though. Like if you work for a defense contractor, you know what your work is for even if you wouldn't know exactly what the product or application was.