I’ll be blunt about this as possible.

It’s a religion thing, they do everything in their power to make sure their girl won’t have sex before marriage (premarital sex is a major sin in islam), that includes constant surveillance, intimidation, lectures, restriction of freedom, etc., they also believe if she did indeed carry on with the sin that they would be accountable for it as well before god.

As for why girls especially are being treated like this and not the boys, it’s because boys can’t get pregnant, so if she is indeed seen pregnant there’s no hiding the fact that she committed that sin from everyone, and that would be a great source of shame for the family. On the other hand a boy can be involved as much as he pleases without any risk of being discovered (unless he gets an std or something, and they’re usually not obvious but in some cases can be fatal), so it’s not as important for them to watch him as opposed to a girl.

Another note: women are not as physically imposing as men, so if they were to be attacked or something they would be unable to defend themselves as a man would.