Are you sure there is anything telling girls they "don't do STEM"? If you spend any time looking into it you'll find there is far more encouragement (and grants) for girls than for boys as far as STEM and related fields go. But that doesn't mean girls think they "don't do social work" or "don't do paediatrics", or that they should prefer STEM to any of the myriad options they are told are definitely "for them". When it comes to the crunch, you can't make them do what you want them to.
And why does nobody care about fields where men are underrepresented? Perhaps if boys were not told that teaching is "not for them" there would be fewer men in STEM. They're not told that though, are they? It's a myth just like "the idea that girls don't *do* STEM". What we need to accept is that boys and girls are different.