how does pregnancy "JUST" happen

ok I'm asking a serious question which I've thought about for a while. there are those people that are on bc and when they feel ready to have a child they go off and hopefully concieve. then there are those people who r either teenagers or adults that don't want a child but want to enjoy sex (that's fine I'm not judging) but then they find out they are pregnant. so I don't get it did they not use protection, condoms break so why risk that, use something stronger. pills are good but you have to remember to take them, so when someone says oh woopsi I missed a pill why r they shocked they are pregnant. there r a handful of people where they can take bc pills on time and they can become pregnant, but that is a rare occurrence. so I'm curious of what you guys think. fro. the way I see it some ppl seem to be careless and then realize uh oh...