Expertise and materials. Casting aluminum is easy, but not very useful. Doing this yourself you can't guarantee grain structure/size, inclusions/porosity, or

Expertise and materials. Casting aluminum is easy, but not very useful. Doing this yourself you can't guarantee grain structure/size, inclusions/porosity, or temper. Plus it's usually plenty easy to mill from billet since aluminum is so soft. Gold/silver is also relatively easy. You need better equipment since the temperatures are 2x higher. Strength isn't important, so casting stuff in gold/silver is quite common. Outside of jewelry there is basically no need for it. Casting something more complex like good steels, or platinum, or nickel alloys- casting will be very difficult. Molds aren't so trivial, you need preheats, and a mistake leads to liquid metal on the floor or an explosion. Sintering can be used with challenging alloys and requires almost no expertise. Everything comes certified and gets heat treated in the oven. If your time is valuable enough to warrant a metal 3d printer, it's valuable enough to make learning an entire discipline too expensive.