It's not just people's embarassment and/or reluctance to get tested. Take herpes, for example. It's very difficult to detect without a breakout, and not everybody who has it breaks out often or at all. So you could easily date someone for 5 years, get married, then get herpes from them. HPV, similar thing. Many people who have it don't show symptoms. Not to mention there are 80+ strains of HPV and only something like 8 of them cause cervical cancer and not all of the 80+ strains affect the genital area (all warts are HPV). You can get HPV without any sexual contact at all, just maybe not genital warts.
Considering the germination period of things such as HPV before symptoms are shown, or HIV before it's detectable by a test, there is little wonder of the prevalence of diseases. It's not just that people don't want to get tested and/or are embarassed, some of it is also the elusiveness of them when compared to the general sexual behavior of people.
Although if everybody got tested regularly, even before being sexually active, we'd all be safer and the numbers would be lower. Of course if folks were less promiscuous we'd be safer as well.
b