Because the economies are interlinked and so a major economy defaulting starts a domino process where almost all debt goes into default. The currency of any country is legal note that says that country will pay that. What happens when the country can't? And other countries are in a similar situation?

Let's say that you're wealthy and bought three houses for three families and they all owe you that money. You don't want a bunch of paper in return, you want to get the value back that you invested. How? If all currencies have lost their value, where will you turn? You'll want something that can't be printed.

This essay talks about it in more detail: https://blankpoole.com

> A major economy defaulting is possible (I put no limits on the ability of Congress to bungle things)

Arguably, even maximal Congressional bungling (at least, absent positive action to force a default, which doesn't seem politically possible) won't make a default necessary, as there are several no-default courses possible without Cobgressional action (“mint the coin”, ignore debt limit due to conflict with mandated fiscal actions, ignore debt limit as unconstitutional)