Why does opening a line test invalidate it?

Tristen • 💙💖💖💙 currently pregnant with baby #5. He's a boy 💙

If you test, and its been dry for a couple hours and then open the test, why does it matter if you open it? Just curious, I see a ton of people asking if they have line eyes or whatever and then nasty ass people freak out on them for " opening" their tests... I know digital tests are not supposed to be opened, but why line tests?