Greece default and why are US States not seen defaulting?
Two reasons:


The United States has a capable federal government that takes care of social insurance transfers and countercyclical economic policies. The American states spend less.
The American states have stronger anti-tax sentiments, and increasing public spendings face opposition from business and rich people.


Take the 2008 crisis for example.

As for 2008, all developed countries, including Greece, had a lot of bad debt in the private sector and a subsequent recession due to the credit crunch. The United States immediately approved two things in response: a $700 bn stimulus and the Fed's QE programs. They cleared bad debt and supported business in 2009.

These measures was done at the federal level. It's the federal government that took additional debt. And the debt looked like this:



(McKinsey - 2012 - Debt and deleveraging)

The EU was slower. And its recovery fund lent money to European countries (as if the US fed government lent money to the states), so increasing their debt-to-GDP levels. This is how the EU, IMF, ECB happened to be the holders of all Greek bonds and a demanding creditor, as for now.

In short, US states got their federal stimulus for free, while Greece had to buy the stimulus on its own.