Thinking about it further (when trying to get to sleep), something came to me, and I decided that it was easier to type than write down to type later.
Maybe lifting the ban on certain taboos that don't need to be taboos just isn't a good idea. Maybe people in general aren't mature enough to discuss peoples' salaries. There are so many assumptions you can make about someone if you know their salary, and with those assumptions come preconceived notions, perhaps based on something that isn't even quite true. Weight... certainly your weight can bring about negative connotations, deserved or not. Age brings even more assumptions.
Maybe these things are forbidden from conversation out of politeness for people who would have to give out information that could be heard or overheard by someone who would use it improperly, or refuse to answer and then look like they have something to hide. You might think that you could know the answer to that question and treat that fact objectively, but the person asked may have their doubts, or have personal issues arising from the commonly held societal views on that fact. You don't ask those questions because it's generally impolite to make someone feel socially uncomfortable, and you don't often know how someone is going to respond. I guess it's even worse when you ask the question and you already know the other person would not want to respond.