I can only conclude that your plastic screen protectors are far easier to scratch than the screens - and if I were selling them, I'd probably deliberately make them easy to scratch!
I chuck my iPhone in a pocket, purse, whatever, and the screen is still lovely. It's a 3GS, bought shortly after they first come out, so that's ...nearly three years? The iPad (original) is in a case, but no screen protector (why bother if it's in a case?), and again the glass is perfect. But I guess you'll never believe until you try, and if you'll never try, you'll just have to take our word for it.