There are probably many reasons. In my opinion, it’s only natural that men are generally more interested in things like speed, engine sound, tyres, technics, aerodynamics and so on. So it is completely understandable that the ratio of men to women in F1 is not 50:50.
However, I am sure that there are some women who have both talent and will to fight against Alonso and Hamilton on the race tracks and who would be excellent F1 drivers (or engineers, CEOs etc.). If Jutta Kleinschmidt could win the Paris Dakar rally, I believe some other lady could do the same in F1. And this is where the social bias sometimes play role as talented girls often don’t dare even to try racing cars although they should.
I believe that there should be more women in F1; however, I wouldn’t be happy if teams hired them only because of marketing reasons. As for the grid girls, I honestly don’t care much about whether we have grid boys or grid girls as that’s not the reason why I watch F1 :)