That is what we are told to believe. Obviously, having people believe in in its truth is useful to women as a whole. Furthermore, women as a whole do band together in a loose alliance to increase their advantage; so few of them disabuse anybody of the notion that they are not necessarily all of that.
Why do people believe that women are wonderful? Probably there are many reasons; here is a stab at some possibilities,
I. Historically they were consider to be mothers, and mothers nurture children (at least they used to). Nurturing naturally enough is considered to be a virtuous activity.
II. The Evil Patriarchy, through reading the Bible and other sources, knew just how women can misbehave. As a result, they set up a system to encourage women to be the kind, sweet, caring, etc. This was much better for them to be for everybody’s sake. Compare this to the semi-feral women that we see of today; one can see why this was as it was.
III. Women for men are pleasant to look at, they are smaller, they are soft; all seemingly indicators of virtuosity. At least if you can convince people to associate these with number II above.
IV. Women have been known to be indirect and manipulative Part of accomplishing this is through posing as nice, sweet , lovable, caring, etc. Once again, the Evil Patriarchy encouraged these attributes for real, feminism encourages the play acting of these attributes for manipulation.
V. At least during the time of the Evil Patriarchy, women did not have to make tough decisions; that was for men to do. Therefore women could stake out warm fuzzy positions on issues, as they were not actually responsible.
VI. Women historically have been vulnerable. As such, they try to minimize threats to them. One way to do this is to spread societal goodies around to buy favor. This is one reason why women tend to favor socialistic policies; always emphasizing that they are wonderful and virtuous for espousing such policies (see, Hillary Clinton)
Once again, people possessing this view of you is an advantage. This is why modern women still encourage this view, even as evidence suggests the contrary. Commenters can add their own ideas as to why it is the way that it is.
NAWALT of course applies