Actually, they only work when they follow the standard physical layer. Try plugging a Line device into a Mic input, or a Line device into a speaker. Or an unbalanced into a balanced jack. It'll "work" but not "correctly."

And, just like saving money with "rubbish DACs", there's room for designing complete crap, noisey audio paths (and noisey amplifiers) in products.

If people could actually agree on a standard (USB3 seems a good choice), sending audio over a digital port seems fine.

The ONLY real problem, is digital audio isn't digital audio. It's a protocol with MANY audio formats. 44Khz. 48Khz. 96, 192khz. 2 channels, 4 channels, 7.1 channels. 16-bit, 24-bit, 32-bit float. Compressed with different compressions.

THAT'S where the real shitshow happens. Because now you're basically in the oldschool Modem situation where you have to try various standards and HOPE that the sender and the receiver both have compatible formats and in failing that, falling back to some standard required format like "stereo 44Khz PCM".