3d printing is not the default fabrication method now that we're getting good at it. It just shines in certain applications.

Getting a little theoretical here

With the current state of the technology, 3d printing lags behind some traditional manufacturing techniques like machining and in terms of speed, cost, quality, available materials, etc. except for some relatively niche cases.

However, that gap is closing a bit every day, it may or may not ever catch up completely or surpass the old technique in those aspects

But if it does ever get close, I could very much see 3d printing being a preferred method

Subtractive manufacturing like machining, by design, creates a lot of waste, all of the chips and off cuts that are removed from the stock are either discarded or require additional energy and/or materials to recycle.

And things like injection molding require custom molds that wear out over time, and can be expensive to design and manufacture

And in either case, you're largely locked into making one thing on an assembly line at a time, and to switch over to a different product you're probably going to need to switch out a lot of the molds and tooling, recalibrate everything, etc. which can be time consuming.

With 3d printing, you could theoretically use only the amount of material that's actually in the finished product (if you design it that it doesn't require any external supports ) you don't need any custom tooling or mold, just generic, interchangeable nozzles (for FDM, LCD screens or lasers or whatever the equivalent is for other printing technologies) and you could switch production from one item to another by just hitting print on a different file.

Again, we're not there, may never be there, but it's a cool thing to think about