I wouldn’t say it is harder to make a modular cable than it is to solder a cable for a PSU. With the modular cables, there are a lot more connectors and connections that have to be created.

Soldered cables. PCB, solder, Wire, female or male pin, then appropriate connector.

Modular cable: PCB, soldered male connections, female connectors, male connectors, female pins, wire, male or female pin, appropriate connector.

There is a lot more parts going into a modular cable.

The expense of the main modular cables you see is the individual sleeving as well. That gets very expensive very quick. For most stock wires, all of the wires are put together, and one set of heat shrink and one sleeve is installed, which is seemingly quick and easy versus all of the single wire sleeving.

Modular is also far more attractive, and makes cable management much easier.

Several time I saw discounts for around 50$, full sets.
That's not too much.