NATO existed to fight off global communism by keeping the Soviet Union out of Western Europe. This was directly in the US national interest because the US didn’t want to face a pan-Eurasian communist front in a future war.
What does NATO do anymore? The US, as the only global superpower with a homeland separated from any potential, conceivable enemy by the two biggest oceans in the world, is largely invulnerable to invasion. And without an enemy with an energizing ideology behind it, what is anyone allied against?