That's definitely a great point. There are very strict federal guidelines on advertising, especially when it pertains to medicine. Taking birth control to prevent cancer would likely not be advised by anyone, and when it comes to cervical cancer about 70 percent of cases are caused by HPV, so the pill is not highly relevant for preventing it. And though studies have shown a direct correlation between contraceptive pills and decreased risks for certain cancers, there is a positive correlation between them and developing other forms of cancer (breast cancer, most commonly).
I personally use contraceptive pills for acne and polycystic ovary syndrome, and appreciate all the added benefits! It's a multi-faceted approach to reproductive health, but it's not right for everyone. And no pill is 100 percent effective against preventing pregnancy, which means that advertising its effectiveness in preventing certain cancers would not be true to everyone as some women would inevitably develop certain cancers (like endometrial and ovarian) while still on the pill/after using it.