There's some air of hidden secret about these articles: Did you know that Edison didn't invent the light bulb! James Watt didn't invent the steam engine! etc etc.
I don't know about you, but I can only invent one thing at a time, and complex systems require a lot of invention. The incandescent light bulb requires electricity, glass, partial vacuum, and very precise tungsten, among many other things.
An efficient steam engine wants high quality steel, tight tolerances, good valves, and no explosions (relief valve, vessel proofing).
A machine gun wants smokeless powder, good belts, many excellent identical bullets with identical powder loads in identical cases with identical primers.
A machine gun needs some way to convert the energy of firing into the cycling of the action to feed and fire the next round.
A machine gun needs to fire when the trigger is pulled, continue firing until the trigger is no longer pulled, and stop firing when the trigger is released.
A machine gun needs a supply chain. It will consume at least 20x more rounds than an individual soldier (30 vs 600 rounds per minute). WWI was an industrial war.
A machine gunner needs a place to stand. A machine gunner wants to be not shot by snipers, not exploded by artillery or hand grenades. A machine gunner will attract attention.
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This merely scratches the surface, of course, but I do wish people had more respect for the complexities of the real world.