This is why it's ridiculous that media in other countries are criticising their politicians for not responding harshly to Trump's tariffs with tariffs of their own.
When America applies tariffs on imports it's Americans who pay them. It affects foreign business slightly, in the form of reduced sales, but the real victims are Americans. When other countries apply tariffs, the main victims are their citizens.
The correct response to someone punching themselves in the face is not to punch yourself in the face.
You're only considering material cost, not time cost of employing someone to operate the machines. Also your system is not really scalable - it would take a long time per unit, making the labour cost even more significant per unit. There's also R&D, distribution, marketing, etc. all before any profit is made. Also, as you mention, the quality of 3D printed pieces would be much poorer.