Asking seriously, not looking for negative responses please

Opening a pregnancy test cassette/ stick:

Why do we say the air exposure will show the indent?

The tests can't be air tight, it's clear that they arent....? I mean they r easy to pop open n eventually dry without opening, therefore no they couldn't be airtight without a seal of sorts, so how could this make a negative a pink line? I understand u may be able to see the test line better but I mean a pink line vs. a line that is NOT pink.

I'd love opinions and answers that are honest, please don't be mean because I asked, I am just very curious