Just look at the myriad failed Kickstarter projects that tried to make consumer stuff. Even getting something normal made in China can be serious work. And far more if you actually want to do a good job of it. Linus Tech Tips put a surprising amount of work into making a screwdriver.

Then consider that the military probably has a bunch of requirements like "this can't break into sharp shards" and "it can't emit toxic smoke if it burns", and they'll want that to be proven. Certifying all those things is a whole job on its own.