>Constructing a rocket engine using conventional approaches can take months, followed by extensive qualification testing to ensure it meets the required specifications. Using a metal 3D printer from German company EOS, Agnikul produced its engine in roughly three days. Agnikul printed the engine out of inconel, a high-performance alloy of nickel and chromium that can withstand high temperatures and mechanical loads.…
https://www.youtube.com/watch?v=kz165f1g8-E
I'm sure that there are lots of problems with this approach, but it is not as obvious as your comment makes it out to be.