Mostly because there’s no such thing as a “digital” signal. A USB cable does not have literal one’s and zero’s flowing through it. It’s a complex square(ish) wave with a relatively high bandwidth along with plenty of switching and other analog noise and distortion mixed in. DAC designers do their best to isolate the analog circuits in the DAC from this noise, but reducing upstream noise almost always pays dividends in sound quality.
This is why Roon Labs themselves explicitly recommend separating Core and Outputs by a network connection in their Sound Quality recommendations. I’m not saying that a DAC won’t work when connected via USB to a NAS or general purpose computer running Core, and neither is Roon Labs. But, in cases where sound quality is a priority (eg., a highly resolving system), connecting a DAC to a NAS will never give the best results.
Many find that there is a correlation between lower transport CPU usage (and associated noise) and better sound from a downstream DAC. If you read Roon Lab’s recommendations for how to use their product for best sound quality with this principle in mind, it may make more sense.
Again, you don’t have to do things the way Roon suggests. But, keep in mind that my audience was 60+ audiophiles at a San Francisco Audiophile Society meeting. All of these folks care deeply about getting the best sound quality they can from their setups. If that’s you too, these recommendations are valid.