You can understand this better from a market perspective. Most people looking to adopt want to get a young child, preferably a baby, to avoid all kinds of possible issues: legal complications from biological relatives, behavioral or development issues from abuse or neglect, etc. Basically these people want to ensure their child has the best odds of having a happy childhood with minimal complications from life before adoption. Unfortunately, these kids are like unicorns. The vast majority of kids who are up for adoption are in that situation because their original parents fucked up badly. These kids are a lot cheaper to adopt in dollar terms but they are expensive in a lot of other ways.
You could just get your name on a list and wait years for one of these unicorn kids to become available for adoption. This is a risky bet. You might never get the chance to be a parent if you get unlucky. For most people this is a dealbreaker; they don’t have the patience and/or lifestyle flexibility to make it possible so they choose to go a more expensive high time preference path.
From an economical perspective you have a market with limited supply and unlimited demand. Combine this with tons of complicating legal and political factors and the price goes up up up.
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