Because they are stepping into the pile of garbage that someone else dumped and lit on fire. Then they are told by their self-righteous elders -- half of whom were so self-centered, navel gazing and narcissistic that they were labeled the "Me Generation" -- that they're too self-absorbed.
lol
Boomers were born into unprecedented levels of post-war prosperity. Gen X and younger Millennials graduated into massive recessions, which will depress their earnings throughout their entire lives.
They were dropped into the worst economic downturn in over 60 years, saddled with epic and unprecedented amounts of debt to go to the schools they needed to be economically competitive. Their parents grew up in a world of near-lifetime employment; that's inconceivable to someone joining the workforce today. Quite a few also
did go to war -- who do you think was fighting in Iraq and Afghanistan? Senior citizens, perhaps?
By the way, we saw fewer soldiers in Iraq and Afghanistan than Vietnam not because of a lack of sense of duty, but because a) our political leaders (mostly Chickenhawks) didn't want to send large numbers of troops, and b) we don't need as many troops to do something like this as we did in the past.
I might add that bitching about the economy and the US is certainly not limited to people under 30. Who has spent the past 40 years moaning about jobs going overseas, about NAFTA, about Japan, then Mexico, then China? Who's voting for Trump?
Why? Because they called themselves that?
The so-called "Greatest Generation" was not so great; they were just like anyone else. They were born into a different world, with different challenges, and they have their own moral, economic and political failures to deal with.
So your answer is... to whine about other people whining? Impressive.