Why does USB feature so much in discussions about DACs when the newer HDMI seems better?

@2psyop how audio file is different from Excel spreadsheet. What is special in 'audio' bit packets over Internet.

@2psyop  the only thing Paul McGowan doesn't say is that I2S was never designed to be transmitted over a cable from one component to another.

Good DACs take USB and galvanically isolate it from the input section, thus eliminating most of the electrical noise that it might otherwise carry, hence USB can (in all but the cheap stuff) be a very good input, even from terribly electrically noisy computers.  In the last few years I2S has made a splash because of companies like PS Audio developing their own "standard" for the pin outs and many other companies have unofficially adopted that same "standard" so they can play well together without the end users being a geek.

I2S has the "claim to fame" that it separates out the clock signal from the data stream and in theory that can allow less jitter and noise.

Note that I2S existed inside digital devices long ago well before HDMI came along. They are just implementing I2S via an HDMI connector now because those were available, relatively cheap, and semi understood by the masses as a "cool connector".

None of my devices so far have an I2S connection so I use USB (instead of coax or Toslink optical). Toslink optical might have been a great digital connection if glass were used instead of plastic and if they had upgraded it to be more robust in bandwidth, but it never was, hence the limitations make it the "worse" connection you can use at least if you are trying anything better than CD Redbook quality bit depths and sample rates.

I’ll take it on word of mouth that in $$$ systems, I2S can often sound better. Overall, it isn’t something I’d worry about. The DAC you choose, speakers, amp, and your room are liable to make a bigger difference in sound quality.

Note, it may be years before "slow as molasses" industry governing boards like the IEEE and their audio steering committees to create or adopt a "real" I2S standard. 

Most streamers don’t have I2S outputs hdmi 

If you have a DDC Reclocker then they have I2S outputs  

the only brand I know that sell true I2S  audio cables without the video wires is the Excellent Tubulus Audio cables.

@2psyop "USB was for data not audio." Sending audio DATA over USB *is* sending data, zero and ones. Digital data transmission does not depend on what type of data it is. Data is data from a transmission perspective.