If you want to talk about adopting preteens/teens/special needs children then it's a different ball game, but that's not what people try to do.
Me and my girlfriend despise babies (for now) and simply don't see ourselves as being responsible/mature enough to raise kids yet. We do entertain the idea though, now and then, and for us the main attractions of having kids seem to be: Something we made together, a "substitute" of each other to live for after one of us dies, and something to carry on our legacy, such as it is.
With adoption, these things can be fulfilled if the child is young enough, to be molded into our image, as it were.
That's not to say that love for an orphan, no matter how old, cannot happen. Of course there can be selfless adoptions, where you just want to make someone's life better without regard to yourselves.
The goal I have with raising my child isn't to create a miniature version of myself, it's to raise an autonomous individual with a robust moral framework. As far as self-interest goes, sharing this experience with my partner is one of the things I look forward to most over the next few years.
But what if the child's own will decides to become an evil overlord instead?
