For exactly the reason johnnyb9 points out up-thread: you don't want to be holding 100% U.S. equities when you're in retirement and the U.S. market crashes.
Target retirement date funds are very different from reverse mortgages. In a reverse mortgage, you're borrowing money, which you pay interest on, which is secured by a large asset you own (your house). The reason they're so dangerous is because you're paying interest on the borrowed money, so it compounds, and can eat up your home equity very quickly.
With a target retirement fund, it's a normal investment which you receive investment returns on, but the asset mix moves from stocks to bonds as you get closer to retirement. The reason for this is that you generally don't want to put any money at risk in the stock market that you can't afford to lose within the next 5 years. Market crashes are unpredictable and often take several years to recover from; if you're in retirement and forced to live off that money month-to-month, you may be forced to liquidate stocks at very low prices, often lower than you paid for them, wiping out your investment gains. You're supposed to move assets from stocks to bonds or other more predictable income streams as you get closer to retirement, so that you know that the money you need to live on will be available when you need it, and can just let the money you don't need to live on sit in the stock market until you do need it.
Target retirement funds automate this rebalancing, dollar-cost averaging out of the stock market as you get closer to retirement so you aren't tempted to panic and pull everything if there's a crash.