I adore Gen Z.
Gen Z didn't believe anything they were told to begin with, because while Millennials grew up in the 90s and early 00s and got some optimism in our culture, America has been a polarized, rapidly-changing nation for pretty much all of Gen Z's time.
The tl;dr I'd say is: "Millennials had our expectations dashed, and Gen Z didn't build up the expectations to begin with."
(Note that I'm not commenting on whether this rapid change is good/bad/malicious/whatever. Just that how we've planned for and lived our lives has changed a lot in the last 15 years.)
We (Millennials) mostly believed what we were told, due to both not having experienced a shock like that before + having more limited information access. Gen Z has enough contrary evidence and ability to share things that they just took a Fast Pass for the cynicism line. And good for them, honestly.
By the way I know this resentment is stupid and unfounded in any reality - its just a feeling I have.
I know this is all generalisation and not everyone born between two arbitrary years are like this - but the general vibe I get from their generation is that the kids are going to be alright.