Its because the Federal Reserve is printing money. Its simply history repeating itself. When a government is spending more money than it has and needs more money, it just prints it. And more money = more inflation. And inflation equals devaulation of our dollar.

1. Our government spends more than we pay in taxes, AND
2. we import more than we export, AND
3. the federal reserve keeps interest rates artificially low.

1. Our voters demand that our government spend more than they are willing to pay in taxes (budget deficit), which increases the dollars chasing the same amount of goods and services, making goods & services cost more, dollars worth less.

Paying no taxes does not make lunch free. What we will not pay for with taxes, we pay for with inflation, the buying power our dollars lose. Those on fixed incomes would be better off if everyone paid taxes.

2. Also, when we Americans buy more from other countries than they buy from us (balance of trade deficit), foreigners exporting to us get more dollars than they want to spend on American goods and services. Just as with the budget deficit, more dollars makes goods & services cost more, dollars worth less.

When the we Americans want more from foreigners than foreigners want from us, our dollars don't buy much.

3. Foreigners exporting to us, but not wanting to buy what we sell, buy dollar denominated investments (for example, US Treasury bonds), for which the US Treasury sets low interest rates (making them an unattractive investment).