I’d guess it has to do with the benefit of doubt based on a lack of evidence, and a dash of confirmation bias thrown in. For example, there’s plenty of evidence that the generations before them suck at choosing leaders and then suck at governing when they finally get a seat at the table. I would choose to defend millennials because as an older millennial I am biased toward that generation. But Gen Z doesn’t yet have a lot of evidence out there in either direction.

All that said, I feel like the statement “Boomers destroyed the planet in essentially every way and made life worse for every generation that preceded and followed it” is just fact. But again, I’m biased.