>but it's frustrating how almost every build requires a few critical pieces that we don't have
This is to add exclusive value to each set. Look at bricklink.com and find an expensive set, and you'll always find one or two pieces that are a unique color or completely unique to the set that are astronomically high in price. The original Millennium Falcon UCS set for example has two grey (I forget which specific grey) ladder pieces to highlight the engine, and those are the pieces that were most expensive last I looked.
This is to add exclusive value to each set. Look at bricklink.com and find an expensive set, and you'll always find one or two pieces that are a unique color or completely unique to the set that are astronomically high in price. The original Millennium Falcon UCS set for example has two grey (I forget which specific grey) ladder pieces to highlight the engine, and those are the pieces that were most expensive last I looked.