These kinds of parts can actually be mass produced. You could probably make 3500 face shields in less than a week with 10 printers. The mass produced Prusa i3 printers actually use quite a few 3D printed parts, and they made over a hundred thousand.
There's just no comparing 3d printing with the correct manufacturing equipment. This machine can probably produce hundreds of face shields per minute (wait until the end of the video for full speed):
