Here's the best explanation...

...I've read so far on what, exactly, would happen if Congress failed to raise the debt ceiling (my emphasis):

The US government is the largest purchaser of goods and services on planet earth.

The government buys everything from equipment for cancer research to metal for warships to toothpicks for federal cafeterias. Suppose the government had to cut 44% from its budget on 2 weeks notice? How sharp a shock would that be to the world economy?

Here’s a comparative. In the worst quarter of 2009, American consumers cut their spending by … not 44%, not even 4.4%, but 1.2%. That 1.2% drop in consumer spending helped tumble the US economy into the worst collapse since the 1930s.

The US consumer sector is even larger than the federal government sector. But it’s not unimaginably larger. US consumers spend about about $10 trillion a year. The federal government spends about $3.4 trillion.

If a cut of 1.2% from $10 trillion was an economic shock, a cut of 44% from $3.4 trillion will be a much, much, much bigger shock.