I mentioned 3d printed guns because, if you don't use off the shelf parts, you really don't have any way of knowing how many rounds you can put through it before it explodes in your hand.

I mean. Ya kinda do. A broken receiver isn’t going to make the gun explode in your hand, it’s just going to cease being functional.

Was there a tiny defect in your print? A misalignment or some debris in the print material?

More importantly, the issue with small defects is sidestepped by good engineering and process control. Your statement applies to almost every sort of manufacturing. All materials have internal defects and a range of potential material properties. You account for this by applying a safety factor so that the design stresses are well below the nominal material properties. For a material with higher uncertainty on the properties you simply apply a higher safety factor

That’s why many 3d printed guns look like bricks when an injection molded receiver would be much more sleek in comparison.

A car has many critical components under the hood, especially when you use an internal combustion engine. That's a bunch of tiny explosions every second, and even setting that aside, you have the transmission, the brakes, the steering, the windshield, the stereo, any one of those could fail and kill you (or make life not worth living, in the case of that last one).

Home built cars have been a thing pretty much since the start of the automobile. People who home-build cars don’t typically build their own drivetrain. It’s just not worth the effort when donor engines from wrecked cars can be had cheaply and easily.

Usually the DIY fabrication work involves building the chassis, suspension components, body panels, etc. As I pointed out, you wouldn’t just print the chassis. You’d use printed parts as jigs to align the welds, build mold for composite pieces, etc.

Nobody is going to be printing a complete car in the conceivable future, but 3d printing can absolutely help you build a good and reliable car from conventional materials with greatly reduced effort and tooling costs.