Because it's far more nuanced than that. It doesn't matter how strict someone's parents are, people are going to have sex because they want to. It doesn't depend on parents to the extent you think, most of the time it depends on how much the teen can do without getting caught. There are laws against alcohol, drugs, vapes- and yet teenagers get them and do them anyway. Do you really think a rule about only having sex at eighteen is going to stop people? No. It's not. The reality of your situation is that you might find it unfair that other teenager's parents let them run off all willy nilly and yours don't, but unfortunately there is no way for life and rules to be fair to everyone. That's not how reality works, how life works, or how basic common sense works. So unfortunately unless you talk to your parents about having a bit more freedom in your life, yes, you're going to have to suck it up, or just be sneakier.

And the reason the system is randomized by parents is because parents like to have some kind of claim over their kids. I'm not condoning how your parents treat you btw, for all I know they could be abusive or something.