Honestly, I've wondered the same thing. So, Moody downgrades credit rating for the US now, even though the debt was already kinda wild before. My take is that these rating agencies are kinda slow and careful, because if they jump the gun and freak people out for nothing, they could seem unreliable. I remember last year, Moody actually changed their outlook to 'negative,' so it was sort of like a warning. But they didn't do the full downgrade until the debt just kept going up and politicians seemed unable to agree on how to fix it.

A couple of my econ friends say rating agencies also get huge flack when they move before it's super obvious, so they wait until things start really boiling over, like now with the $36 trillion debt and higher interest costs. Moody's waited to see if the US could get its act together but, since nothing changed, they finally said alright, risk is up, time to make it official. It's like waiting until your grades actually drop, not just when you start slacking off. Definitely feels late, but I guess they're trying to be super careful.