Despite the fact that "there are so many kids that need to be adopted", the adoption process is long, tedious, and most people who want to adopt are not actually qualified to do it. It's not like adopting a pet where pretty much anyone can do it, they actually have standards.
Not only do you have to have steady income (and make over a certain threshold), but you have to go through a background check, have social services inspect your home to make sure it meets their standards, take something like 50 - 100 hours of training courses, then once you are a "certified adoption household", you are put on a waiting list for a child. You can be on that waiting list anywhere from a few months to several years (more if you are looking to adopt a newborn). Then once you have the adopted child, you need to have regular check-ins from social services to make sure the adopted children are being taken care and are happy. Anything looks fishy and the kid is taken away and put back into the adoption system.
Conversely, what does it take to have a child naturally? A woman being pregnant for 9 months and 24 - 48 hours of labor. Not to undermine child birth but that does sound a bit easier don't you think?
Then people like Angelina Jolie get to adopt instantly due to money. ---