I am not defending the US government but the reason why you hold treasuries is because if you want to buy something in two years, or ten years, etc. there is a good chance you will want to buy something with dollars. Oil, soy beans, New York real estate..
In a way, the bond market tracks international central banks' confidence that this will continue to be true. If they lose some of that confidence, their stock of bonds moves from USD to RMB/EUR/JPY/AUD/CAD/SGD/etc.
Until you can buy oil without dollars. This was an explicit agreement called the Petrodollar system, which was premised on the US navy securing international trade. If the US navy proves unable to do that (and exhausts its arms and demonstrates unpreparedness for future conflicts) then the whole premise falls apart.
You might also stop buying real estate if the government seems unstable and economy is poised to collapse. Good soybeans also depend on fuel prices and the continued flow of fertilizer...