Why is the salt washed off the cabbage in kimchi but not on sauerkraut?
The initial salting draws water out of the cabbage, otherwise the kimchi tends to get soggy/mushy.  As you mention, it is frequently rinsed off after the initial salting.  If you listed the ingredients that follow, I think you will see that there is quite a bit of salt re-introduced.  The recipe I use, for example, includes, soy sauce, fish sauce, and salted shrimp, among other things...this creates the brine necessary for lacto-fermentation and preservation.