A lot of the really expensive cards are also foils, which for the card stock they use for English cards ends up warping quite a bit over time. I knew multiple people who refused to buy foils ever because of this.

Happens to other collectibles too. There are some Games Workshop miniatures that are made in resin, and you can tell the clones appart because they use far better resin, which bends less. For some old plastic molds it's the same thing: You get much better cast out of bootlegs.