I don’t think there’s a technical reason why separates would be better, in fact if we can remove one cable (say the USB) from the equation you have an easier time controlling all issues related to jitter.
Having said that, there are good reasons for keeping them separate. Being able to pick the sound quality of the DAC, and the software that is used to stream separately are really nice for me. For instance, I only use Android control devices, some famous streamers are Apple only. Next, what happens when a new streaming service comes along and you want to keep your DAC but the streaming software is no longer being updated? Replacing a relatively inexpensive streaming component down the road means I can keep my DAC investment.
In my case my streamer is a Raspberry Pi based device, around $200 with the fancy power supply and case. If my software is no longer adequate or I wish to use an unsupported service I can swap it out without issue. The same for network features, such as Wifi 6, etc. In these cases having a separate streamer makes your DAC future proof.