Why People Treat You Like You're Incapable

When some people keep criticizing you when you do certain things, it may be because they don't want you to do that thing, period.
They don't want you to be proficient at it, or gain confidence in your ability.

John goes down to the ball field to practice. Several other kids are there practicing too, with a couple adults. When Bill gets up to hit, everyone says "go Bill, you can do it, nice job", whether he gets a hit or not. When Jeff gets up to hit, everyone says "go Jeff, nice swing, good job", even if he doesn't get a hit. When John gets up to hit, only a couple of people say "go John". John has an excellent swing and gets a hit on the third pitch, but instead of saying "nice hit, John, good job", one of the adults tells him what he did "wrong". The other kids hear this and chime in too, giving John all kinds of criticisms about his stance, his swing, even the way he runs. They call these "pointers", and "constructive criticism". Reality is, John is one of the best hitters, and the adult is jealous. John is a better hitter than his own child, and seems more skilled than he was at the same age. With enough of this focused, contrived "constructive criticism", John will eventually stop coming to practice, lose interest in baseball, feel like he doesn't fit in, or stop believing that he has any reason to believe he could be good at baseball.

When Susan picks up a hammer and a saw, she's in Heaven, she feels creative and at peace. She loves to build things. Every time one of the people in the neighborhood see her building something, either another kid or an adult, they try to pick out something she's doing "wrong", even if they know very little or nothing about carpentry. They give her all kinds of advice that she doesn't ask for, and they "assess" her work. They compare things she builds and makes to things their uncles, fathers, or grandfathers have made. The only person who does not constantly try to pick her apart is her brother, but his friends all do it every chance they get. Eventually Susan starts trying to hide her projects, but when someone hears her sawing, banging or drilling, they will either make a comment or find her and comment on what she's doing. If they can't find something wrong with what she's doing, they will pick on her clothes or her hair. If that doesn't work, they'll say what she's doing is dangerous. If that doesn't work, they'll try to criticize the actual equipment she's using. Every criticism will be denied, or called "Constructive criticism". The invasion of her space will be denied and laughed off, and if she stands up for herself against their behavior, they will criticize her for that, too.
Really it's a campaign to stop Susan from doing carpentry, because everyone in the neighborhood wants carpentry to be a "male" thing. They don't care about reality, or Susan, obviously. All they care about is trying to force her into fitting into their stereotypes that comfort them. Susan's proficiency and enthusiasm destroy their illusions about males and females, so they target Susan and try to make her STOP. (Hopefully, her brother will remain a solid friend to her and keep supporting her and seeing her for who she really is, and not give in to the peer pressure from his ignorant and immature friends. She will need his friendship and support, otherwise she will probably give up due to the stress of the constant harassment, and give up the very thing that brings her peace and confidence.)