Media influence; note their concentration on millennials specifically. The media loves to obsess over the misdeeds of the terrible millennials and boomers. My personal theory to explain this is that ~everyone currently in a position of editorial power in media is gen X; note that the media never mentions gen X.

However, change is coming, as the gen Xes will start aging out soon. Within about a decade, expect the media to be decrying the villainous gen Xes and Zeds, as the top editorial jobs go millennial. Constant memes about cereal will replace the stuff about avocados, etc (Gen X are the peak breakfast cereal generation, as they were largely around before truth-in-advertising rules severely restricted manufacturers from claiming that the product containing a lot of sugar was good, actually).

This isn't new; "everyone significantly younger than or older than me is terrible" has been a meme for, at least, thousands of years. Shows up in ancient Roman and Greek stuff, say. The _degree_ to which people think that Other Generations are weird and bad may be growing, though, as the media writ large becomes more pervasive.