Impossible to answer without knowing what you mean by "so common." The rate is around 1-2% of pregnancies [0]. I wouldn't consider that very common. If everyone with a good reason (health risks, life circumstances) to get one did so, I'd honestly expect more.

Furthermore, birth control is not 100%. You can do everything right and still get pregnant.

0: https://en.m.wikipedia.org/wiki/Abortion_statistics_in_the_U...