My experience as a recent female STEM graduate:
A lot of outreach does target middle school girls, but it's slow-going. You have to have parents who are interested in seeking these opportunities out for their daughters instead of gymnastics or ballet or what have you. I worked at two of these camps, and 90% of our students had engineers for parents (meaning they were already exposed) and 100% had college educated parents. High school and college students are already actively seeking and can choose for themselves.
Plus, we have to encourage development for women as best we can now, not wait 10-15 years for a middle schooler to start her company. The development of female-driven apps and services like ClassPass, Pinterest, and mobile games reaches the middle school market and inspires them the way video games have inspired hundreds of guys.
Once you're actually in the program, there is a high attrition rate because of the gender imbalance. In my own experience, I have been assumed to be a recruiter at career fairs, had my work done for me out of chivalry, had guys - even decades older guys at my internships - try to chat me up at work (making it difficult or impossible to be comfortable in work, class or group projects). When I went to interviews, a number of them told me "You're so social! Have you looked into technical recruiting?". I thought this was a nice way of saying my technical skills were not up to par, but when conferring with my female classmates it was just a common refrain given to "girly girls".
It's easy to be told to "suck it up", but feeling out of place over and over really gets to you. It's not a good learning environment. The extra scholarships and support for women give them encouragement to keep going. Without my SWE group, I'd probably be a technical recruiter instead of an engineer right now.
Diversity is important. One of my friends is a Product Manager at one of the big tech companies in Silicon Valley. At one meeting, the company nearly threw out a very popular feature - because the feature was popular among women, and the people making the decision were men and didn't realize it. STEM doesn't guarantee success, but from a company's perspective diversity among employee perspectives means targeting a larger number of consumers.