Also it should be said that pregnancy isn't the expected outcome of sex. It occurs in much fewer than half of sexual encounters. And many women get pregnant that were using birth control. Birth control methods vary tremendously in effectiveness, and typical use is much different than perfect use.
As Judith Thompson put it, it is better to think of it like pollen. One can close their windows and buy mesh screens to block out pollen, but by the law of large numbers, even the best defenses can fail.
Nor should we ignore that in some cases, people have sex under less than ideal conditions of consent.
A majority of women that get abortions were using contraception in the month they got pregnant. For the vast majority of women, it is their first or second abortion ever. For the subset of women that have repeat abortions, they tend to already have families and usually were using birth control that failed.