I think the main reason why there's no salt in the water is simply because it's ineffective as seasoning. If you want the noodle seasoned, then adding salt into the dough or the sauce is much more effective.
Also salt in China, especially in the inland part where noodle is "the food", was historically simply too expensive to be used as anti-binding agent (see below about cooking method). This is different from Japan or Italy as nowhere in Japan or Italy is particularly so far from the sea that the availability or price of salt is ever a concern. When modern days comes when salt is dirt cheap, the Chinese are already used to other anti-binding techniques.
Personally I throw dry noodle in the pot in very small batches and make sure the they cross over each other as much as possible, then stir quickly for a few seconds, problem solved (key is to weave the noodles at greater than 30 degree angle. Separation doesn't help if they are parallel and they still bind very strongly).
To your other question, I think traditionally, or historically, noodles isn't something cooked to the individual order. The most civilian way of cooking noodle is to boil 10+ servings of noodles in a few gallons of boiling water, then chill and temporarily store all the noodles in cold water(so that they don't over cook when the cook is serving each order), then serve each order by adding different toppings.
To cook noodle in the sauce renders each order a one-off operation and not part of a batch. But, if you order a noodle in a slower paced restaurant where efficiency isn't the top concern, there are variants that is cooked in the soup or sauce, such as Hui mian(烩面).
In this regard, noodle is very similar to dumplings: the mainstream cooking method of dumplings also favors efficiency by cooking in water in large batches, but variants that is cooked in soup to individual order also exist.
The last reason I can think of is, Chinese or also Japanese seem to prefer smooth and chilled noodles than hot and sticky ones, so they tends to use much more water than a soup can provide in order to wash off the starch during cooking, and chill the noodle with even more cold water. I myself prefer smooth and cold noodles than sticky and hot ones. This is why even noodle soups like Tonkotsu Ramen you never find the soup to be steaming hot. This could be yet another reason why the in-sauce versions are not the main stream.
Last last thing, for zhajiangmian(炸醬麵), it's simply because the sauce is too dense in flavor (strong in salt) and short in water. It's just plain impossible to cook noodle in such a sauce unless you dilute the sauce, which isn't a great idea.
