Because the Boomers saddled them with massive systemic debt . . .

Why Millennials Can't & Won't Grow The US Out Of Trouble

. . . college debt, gutted the economy, forced massive new, and highly destructive regulations on them? 
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More below.

"Based on the generations relative population growth vs. total population, the Boomers added an average 0.8% annually vs. the Millenials which, on average, added 0.15% annually... or about 1/5th the impact the Boomers had on growth.  Really simply, Millennials require far fewer new houses that did Boomers, far fewer new cars, far fewer new factories to produce the new goods, fewer new suppliers, far less new infrastructure to support them, etc. etc.  The virtuous loop of growth creating growth is an absolute minimum.  Millennials economic impact, by the numbers, is nothing like that of the Boomers now nor will it be akin to the Boomers over the coming decades."

Why? Well for starters, the Boomers have forced idiotic changes to the university system including a massive increase in administrators, silly SJW BS, and all of this caused costs to skyrocket, and concomitantly Millennial college debt to skyrocket. Hard to buy anything when you already have a home mortgage level of student debt.

Also, the Boomers are only paying about 75% of the taxes necessary to pay for the government programs and goodies they want. This means the rest is accruing as debt, which the Millennials will have to pay off. So just about the time they pay off their student loans, a massive tide of federal debt will wash over them. 

We are just getting started. The Boomers also have kindly failed to adequately fund Social Security, and about 50% of them have ZERO saved for retirement. The Millennials will need to pay for this fiasco as well. 

Still just getting started! What about pensions, luckily the Boomers have pensions some public, some private, so there is that. But, alas, there is no money in either the public or private pensions. They are roughly broke having spent the better part of the past 20 years defrauding their pension recipients of value by inflating their expected rates of return, among other fraudulent accounting procedures. The Boomers expect the Millennials to pay for this as well.

Medicare? Fubar! It is bankrupt, and that will soon be obvious, Boomers have simply punted not paying what is necessary, leaving it to the Millennials.

Then there is the state debt crisis, again, this was a Boomer created problem, which they intended to leave for the Millennials.

The Millennials have a solution to all of this generational theft. They work at reasonable jobs, or reasonable pay, own reasonable homes, or rent reasonable apartments knowing that no matter how much the Boomers want them to pay all this money, they will be unable to force them to pay. The Millennials do not have enough "other peoples money" to allow the Boomers to scrape off enough to make this hellish system work. If the Boomers take more than a small amount more, the Millennials will enter the poverty ranks, qualifying for welfare, and all but eliminating their tax payments. 

Checkmate! Expect the generational theft to backfire badly on the Boomers. 

Some related reading:

The Business Insider finally finds some angst about private pensions