The risk for credit default is very close to zero, but the risk for renegotiated contract terms is not zero. That's what the whole Mar-a-Lago Accords was about. It is a strategy that describes in detail how that would be executed. The likeliness for which is up for debate, of course, but it's certainly not a risk free asset.

Add to that the rampant debt increase over many decades and zero political will to rectify the situation, which is why the rate of return is so much higher than in more politically stable countries such as for example Switzerland, Germany or Sweden.