[pissant]

Or conversely, how come so many adopted kids find out they were adopted and find their birth parent(s)

[RummyChick]

It isn’t as easy as you claim...and could very well take a court order.
http://encyclopedia.adoption.com/entry/sealed-records/319/1.html

[El Gato]

Or conversely, how come so many adopted kids find out they were adopted and find their birth parent

The records can be unsealed by Court Order. Courts are increasing wont to do that, at least on a case by case basis.