Socioeconomic status, religion, and education all factor in here.
Religion can be key. Some religions push really heavily for children and some push really heavily against contraception of any sort. That's when peer pressure can come into play and if you're involved in a church where everyone is having children and you're being pressured to have children, some people may eventually give in because they want to feel worthwhile within their religious community. As for birth control, when you're guilted into believing using it is a sin, you aren't just going to stop having sex with your spouse/significant other, so pregnancies are bound to happen.
Education is another thing. Okay, it can be given that the vast majority of people understand how the body works and everyone knows that sex can lead to pregnancy. But that said, not many people understand how menstruation cycles work (if you aren't using birth control) and so they aren't having unprotected sex responsibly because they just don't understand or know how. A lot of religions (and for a long time, public schools and conservative communities) pushed teaching abstinence instead of responsible sex practices, so people just don't know how to go about gaining access to birth control. It's also been statistically proven that the less educated people are, the more likely they are to have children (because of these factors and more).
Lastly, their socioeconomic class plays a heavy hand. Some people can't afford to regularly purchase birth control. They can't afford to have the medical procedures to make it impossible to have more kids. Birth control can be expensive and it's unrealistic to assume that people in relationships will just abstain from sex entirely "until they have enough money" (which could never happen, as when is enough, enough?).
Some people handle having children very irresponsibly and they know it, but a VAST majority of the poor people who have several children just don't have the access to the same things wealthier people do, so they can't help themselves the way wealthier people do.