You're very privileged to have grown up in a social setting that rewards you for your assertiveness. Not everyone, and especially women, has had the same environment and socio-cultural expectation of them. It'd be lovely to foster a culture in which these issues weren't issues - but for now we have to address the fundamental differences in how we treat women and girls, as far as assertiveness is concerned, from how we treat men and boys.
Actually, I was bullied by a caretaker and certain peers when I was a kid and had anxiety related issues as a result. After abusing drugs and neglecting myself for a long time, I faced my problems and learned to stand up for myself. Way to hold a prejudice against me based on my gender, genius.
There's no need to be rude. Everyone has different experiences. Not everyone is assertive or has the opportunity to learn to be assertive. Especially when social expectation is against them.
You're missing the point of what assertiveness is. It's about creating opportunity, not sitting on your ass and waiting for the world to cater to your needs. Life is what you make it and one has nobody to blame but themselves for where they go. True, we don't choose the starting point, but we do choose the destination.
The core of assertiveness is disregard for cultural expectations. Girls are rewarded for meeting unhealthy expectations, while we boys are punished for failing to meet unrealistic ones, and seeing that and the widespread lack of sympathy is more effective at eventually enabling many of us to start saying "fuck that".
