And for all those 2 million couples, there are plenty of foster kids out there, but they're not as "desirable".
One thing to remember on that, is that many of these looking to adopt are young and/or first time couples.
Just like there is a growing and learning process for children, there is a growing and learning process for parents. Parents learn how to parent their children as they grow. Ask any foster parent and they'll tell you that a first time parent needs to start with a baby, not a five or eight year old child. Those children need to go into stable homes with parents who already have or have had children. Unfortunately the player pool there is much, much smaller than parents looking to adopt their first child.
Regardless, the issue of the existence of foster kids has little to do with abortion. Very few of those kids, if any, were put up for adoption instead being aborted and just ended up in the system. They were put into the system after the fact.
My cousin and her husband have had an interesting trip down this road. After a few years trying to get pregnant, injecting hormones and the whole lot, my cousin was told that she would never be able to get pregnant. So they started looking into adoption. They wanted a baby, but the waiting list was almost like the one for Tide Pride. They decided to try a program that let them become foster parents, well, let them try and decide if it was something for them. They got a couple older kids (4 and 7, I think) who needed a home while they were being processed into the system. They spent several months with them, until a more permanent home was found. They could have had the option of trying to adopt or keep the kids as foster children, but they decided that is was too much too fast for them as parents. So, they decided to try and wait for a baby. After about two years of waiting, my cousin woke up pregnant. The doctors told her it would never happen, but it did. They're going to look at foster or adoption down the road, as they do want more than one child and can't count on getting pregnant again. However they, as most first-time parents, need to go through the process of raising a baby before they'll be able to handle an older child coming in.
If you ask social workers they'll tell you that while they appreciate a couple wanting to get a child out of the system, first-time parents rarely work out as foster parents of older children for that very reason. They're just not ready.
Oh, and if half of the 1 to 1.5 million babies that are aborted each year were instead given birth and placed for adoption, it would still be years for the waiting list to catch up, assuming more people don't sign up to adopt.
Personally, I know at least a dozen or so families who would be willing, who would want, to adopt an infant if there was availability. They haven't signed up for adoption because of the waiting list. But they don't show up in the numbers wanting to adopt. There are millions and millions of people wanting to adopt an infant.
And there are a million or so babies that are denied life every year simply because it's not convenient.