I've worked in the plastics industry. A lot depends on the kind of mold of course. A mold can be somewhat complex when all is said and done, and made from a combination of parts that have to be ordered, or materials that are difficult to work such as hardened steel. Producing multiple pieces that fit together correctly and aesthetically can be an art. Whether the mold actually works in your machine without problems depends on the quality of the mold maker.

And it could boil down to the fact that mold making is a mature industry with a lot of established supply chain and lead time expectations, and nobody is trying to rock that boat.

Of course you don't want to spend all of that time to get a mold that you can't use because you screwed up on your end and forgot to add a needed feature. The world has not moved away from the benefit of a physical object that you can hold in your hand and see that it's actually right. And the 3d printed prototype can also let you check other things while the mold is being made, such as the fit of things like circuit boards, buttons, and connectors.

Molds are surprisingly complicated. They weigh 1,000 pounds or more; some molds we have weigh 10,000 pounds. They're made of high grade tooling steel that must be annealed to the right hardness and ductility. At each injection, they are subject to pressures of 25,000 pounds per square inch, and this happens every ten seconds 24/7 for the duration of the run (which can be weeks or months). Over time, plastic cuts into the steel, like water on rock in a canyon. So it takes lots of milling time, because tool steel can't be cut fast, like wood or aluminum.

The molds also have to be machined to high tolerances. Take vents as an example. Vents allow the ambient air inside a mold to escape as plastic fills, but they must not allow the plastic to escape out the mold on the same route, so they must be machined to a tolerance of around 3 tenths of a thousandth of an inch. That in turn requires high precision CNC machining over fairly large spans, and with many subassemblies to make sure the two halves fit together just right.

Some kinds of molds can be completed faster. Usually smaller parts, or parts where a base size of a mold has already been completed and you are just using inserts. Or if the mold is not expected to last very long, it can be made out of weaker material, which also cuts faster.

I think there are some interesting software opportunities to improve mold build lead times. I suspect some of the design of the molds could be more procedural and automated than they are. That would help cut down the engineering time. And some people are exploring 3D printed molds, which is a cool idea, but currently they are very weak and not useful for production because nobody yet has figured out a way to 3D print high grade tool steel.

3D printing has actually likely increased the number of mold designs we're doing, because it makes it easier to innovate and iterate, and easier for a customer to feel comfortable with a design and move forward with it. It reduces customer utility and engineering risk, and so cuts the total cost of product development.