Sooo Why cant unplayed sims make their own decisions (Marry,Children and Dating)

It's probably because when you're playing with a family, other sims just simply aren't loaded in. As far as the game knows a that moment, only your played sims and the sims walking around in the neighborhood exist. Because of this, the game simply can't continue to simulate the other families, because for the moment they don't exist.
Though as far as I know, the game should be populating empty houses. I believe you can turn this on somewhere in the settings (though I'm not sure).