USB is the defacto-standard. Anyone who wants to be in the marketplace needs to follow the market trends. It is not the best transport, but it is what we have.
PCM, coax or fiber, relies on the HOST to providing clocking information. Notoriously poor. USB uses the DAC or STREAMER internal clock. It can be as good as the designer wants. For $100 it can be excellent.
Some new higher end DACs also buffer and re-clock the PCM, so it would them be just as good as the USB Mode 2.
Stupid engineers? Yes, if they do not design their interface to deal with the realistic world. An AMD galvanic isolator is a 50 cent chip, yet why don't $5000 DACs include one? Incompetence!
I will say ONE MORE TIME, I measured the noise injected into the analog output using "free-be" cables which cut open were very poorly shielded. I measured none with well made to spec. I measured none using a $12 external galvanic isolator.
Just trying to provide factual information. Not everyone has a technical background so they may be more susceptible to snake-oil or may more easily fall into placebo conclusions. A little knowledge helps.