Why do some people think engineering is not for girls?

I think in some ways this is a chicken-and-egg question.  The stereotype of the male engineer is perpetuated by the (unfounded) idea that men are better suited to more analytical professions, which trickles down in society and is taught (either consciously or sub-consciously) by parents and teachers to children.  The added stereotype that engineers are nerds, coupled with girls' inherent desire to be popular, is another stumbling block in kids' formative years.  

As with any other stereotype, the best way to shift things into balance is for those of us who don't fit into the stereotype to make ourselves examples of what we want to be the norm.  Be the change!  No matter your age, get involved in your community and share with younger women and girls that being an engineer is cool!  You get to work with awesome tools, build amazing things, shape society and provide valuable services to people, all while getting paid handsomely for your expertise!  

I've found that my participation in the Society of Women Engineers (SWE) and other organizations that promote this world view has been beneficial for my own socialization and for me to share my experiences with others to encourage them to do the hard work that is necessary to be successful in an engineering career.  We don't have to be defensive or militant about our cause - being a successful engineer and sharing your story are all that is needed to make a big difference!