Their hormones are raging, for one thing. They are discovering their sexuality, and it is like driving a big, powerful car without having had any lessons. And of course, consequences are something that can never happen to you when you are a teen.
I do think there are a few dynamics that contribute to the teen sex and pregnancy aside from this, though. One is that the onset of puberty seems to be coming earlier and earlier. So, bodies are developing way ahead of minds. A second is that many parents seem to persist in the belief that if they don't talk about sex with their children or allow them to participate in good sex education in the schools, sex will not happen. And that clearly is incorrect. A third reason is that society seems to accept this behavior, or at least, it seems to be accepting of the consequences, i.e. pregnancy. There no longer seems to be a sense of shame associated with pregnancy out of wedlock, and this is clearly reflected in the behavior of popular culture, singers, actresses, and sports figures. A fourth reason is that there is a segment of the population that opposes birth control and/or abortion, so the means by which pregnancy could be prevented or eliminated is not available, particularly since so many states are making it difficult or impossible for teens to avail themselves of either without parental consent. Better, I suppose, to ruin one young life to save one that does not yet exist.
I lament these changes, and i have personally always done everything I could with my own children and with students to emphasize that sex is something that should be mindful, responsible, and loving. If I have saved one teen from a bad choice, I am happy.