Why are so many young girls getting pregnant?

This topic gets me so mad every time I hear about another younger girl freaking out she might be pregnant or is pregnant. They'll say "Oh we didn't use a condom and he accidentally went" or pulled out or whatever the reason may be. Why didn't you use a condom? Why didn't you buy them if you were out? You can't honestly sit there and tell me you didn't THINK you'd get pregnant? I would just like some insight on why this is an unfortunate trend and why so many of these girls play dumb about it?