This is anecdotal, but in my experience girls just genuinely aren't as interested in STEM or any sort of challenging degrees for that matter. The girls I knew all wanted to have their weekends free to go out drinking more than they wanted to get good grades. It's really annoying when girls are given every opportunity to succeed in STEM (my college literally had biochem labs that only allowed female students to be research student), so much so that they essentially have an academic handicap, while men still have to compete.