Why do girls get pregnant at 18?
Lack of education, cultural peer pressure to have something as status, desire to have someone who will "love only them", lack of available birth control, lack of parental supervision.
pit bulls bite
usually from having sex
The same reason they get pregnant at 12 or 40, unprotected sex. If you mean why are they having unprotected sex at 18 years old when they know the possible outcome then I presume they don't care.
Because they have sex and either don't take proper precautions to avoid pregnancy, take precautions but they fail, or they want to or don't mind if they become pregnant.