"What about all the orphans in their own country?"
In USA, for example, you face: (1) typically mandatory open adoptions (e.g., you babysit and the birth mother hangs around; (2) many/most kids up for adoption are special needs kids, drug addicted(and it takes a very special parent to manage that); and (3) the constant possibility that the mom gets out of prison (or whatever), shows up, sues and demands her kid.
Or the like.
My sister went through this, replete with drug-addled crack whore showing up years later demanding money or she'd take "her child" away. (My sister moved away, the family changed their name our family name, and basically lives off-the-grid now, far, far, away.)
Foreign adoptions are generally much more "final" and "closed."
That, and many, many people are especially moved by the disgusting fate that awaits Chinese girls in these orphanages if not promptly adopted.
You left out the worst possible scenerio. The mother named the wrong 'daddy', and some guy shows up a few years down the road, claiming his rights weren't properly terminated, to extort money.