Because 85% of the time or better, the two go hand in hand. We'll have to agree to disagree on the remainder of the abortion thing.
Also, you are assuming that the woman would agree to adoption in the first place, then take a pregnancy full term, see the kid squirt out, and still have the ability to allow it to be sent off to someone else's home. If she changes her mind, she does so not only for herself, but for Fetus too, and thus is not a very good option for Fetus's future (pun intended).
