All those depressed couples trying for years to have their own baby, spending thousands upon thousands of dollars on infertility treatments, not to mention the counseling and emotional baggage...parenting my kid? NO THANK YOU.
Adoption seems to be part of plan A more and more these days (and not just because the stars started doing it). I have heard a few young couples mention that they don't plan to have bio children--they feel the planet has enough people, and some of these people are children needing families. One of the guys even said he didn't think his genes were so special that he needed to reproduce.
Also, many families today were built through adoption--adoption only, or bio + adoption. To people raised with adoption as part of their family/extended family, it seems like a normal thing to do--I was raised with cousins who were adopted and I always assumed I'd become a parent through the bio route and through adoption. That's how it worked out, too.