I think vendors being open 24/7 was a quality of life choice. Different planets work on different time-scales. In skyrim, you fast travel from Riverwood to Whiterun, and it only takes a few in-game hours. You leave Riverwood at day and likely load into Whiterun at day as well, so shops and quest-givers are more likely to be up and open.
In Starfield, the day/night cycle and the distances are so different and vast that every time you jumped anywhere it would be a 50/50 on it being night and you having to find a bed or chair to wait or not. I think that would get tedious, so the shoddy solution is that everything is open 24/7.