rationalfrummie- this is the answer I got over the years when asking the question, but, in truth, it is not satisfactory. There are many halachas that ae ‘self evident’ and they still codify them in shulchan aruch.
daas yochid: well, would you take down the mechitza in a shul when there are few women? Of course not- so you cannot simply say that the mechitza is based on the bais hamikdosh.