First, lots of Treasuries are held by those other than US lenders. They're held by foreign governments, private citizens, non-lending corporations, etc. As of February 2015, $2.1 trillion in Treasuries were held by US commercial banks:
http://www.bloomberg.com/news/articles/2015-02-23/bofa-leads-charge-into-bonds-as-banks-build-2-trillion-hoard
That's compared to $18.4 trillion in debt. So you're talking about 11.4% of the picture. The other 88.5% of purchasers are opting for Treasuries, even at ultra-low rates, over a whole planet's worth of assets they could choose from. Why? Obviously, it's because they think those Treasuries are unusually safe, relative to their other options, and so they're willing to accept very low return.
Even when it comes to the lenders, nobody is forcing them to make loans. If the only way to make a loan were to put money at risk in the form of Treasury reserves with a high default or devaluation risk, lenders would either stop lending, or would at least demand high interest rates on those loans, to cover their perceived risk. But that's not what's happening, obviously. Right now I can go out and get a 30-year fixed-rate mortgage at 3.592% APR! That's insanely low. Not only does that indicate the lenders think I'm a low risk, it also means that they perceive those US Treasury reserves as a low risk, too.
What makes you think that? Traders choose dollar reserves because they're seen as stable and safe, but they can choose other options. Russia was recently thinking of switching to Swiss Francs... not because of any perceived monetary risk of the dollar, but because they're more vulnerable to US sanctions when they're holding a lot of dollars. There have also been others who shifted to Euros, including Iraq ahead of our invasion. The Iranian oil bourse works in currencies other than the dollar, as well. You can trade in Euros, Yen, Yuan, Rupees or a basket of currencies, but not the dollar.