As a simple matter of fact, abortion is birth control and is always used as such. The whole point of an abortion is to terminate a pregnancy before you get to the birth. But if your point is that it's better to stop the process earlier, by way of contraception, I agree.
I guess an analogy would be to having a cancerous skin growth removed. Obviously, it would be better if, instead, you avoided ever getting it in the first place by wearing sunscreen when you go out. But if that ship has sailed, getting it removed is still a way to prevent metastasis. Or, if you prefer, it's like liposuction, gastric bypass surgery, or weight-loss drugs, to deal excess weight that would better have been prevented from happening in the first place by way of healthy diet and regular exercise. Or statins and insulin to deal with high blood pressure and high blood sugar, respectively, when each likely could have been prevented entirely by smarter decisions earlier in the process of developing those diseases.
In that context, we can think of the anti-abortion crowd as analogous to if there were a religion that said that providing those kinds of late medical interventions for obesity, melanoma, hypertension, and diabetes just lessens personal responsibility, and so those people should be forced to suffer the consequences of their choices.