When there is no danger that the U.S. will do less for them, allies have no incentive to do more.
I have long suspected that dependence on the U.S. is exactly what the U.S. wants for its allies as it recently conclusively demonstrated by blowing up the Nordstream pipelines.
I agree with all of this, but there a few additional points to be made. The "real" reason why NATO was preserved after the collapse of the Soviet Union was that France and the UK didn't want to be left "alone" in Europe with a united Germany. The expansion of NATO into eastern Europe, driven by Wilsonian idealists in the Democratic Party and militarists in the Republican Party, acquired a disastrous momentum of its own as the nations of eastern Europe were unsurprisingly happy to accept the U.S. as an alternative to having to choose between Germany and Russia. Our "irrational willingness" to fund all this is quite "rational" if you think of it in career terms for both the American military and all the think tanks and university programs centered around international affairs. As Congress has repeatedly demonstrated, they love spending money on defense as an end in itself, and, like the Pentagon, are always on the search for a new enemy to justify their pork. Think tank funding isn't nearly as lush as defense, but it's definitely "healthy". As you may be aware, the Johns Hopkins School has acquired what used to be the "Newseum" and is fixing it up to be a monument to Wilsonian idealism/interventionism. There are thousands of bright, ambitious people in the military and in those think tanks who want careers and are passionately invested in the "fight to do right". It is sad to see intelligent, idealistic people like Anne Applebaum and Garry Kasparov surrender entirely to the notion that Ukraine is a replay of WWII with Putin as Stalin/Hitler. World War I, in my opinion, was caused by the unwillingness of the aristocratic military elites in Austria, Germany, and Russia to accept that they no longer had a reason to exist. I fear our military intellectual elite could lead us to a similar disaster--one that would encompass the entire world, rather than merely a continent.
Empire, how does it work? The neocons claim to be great students of Ancient Athens and assure us that they are aware of the Thucydides Trap, but nothing in their actions suggests that this is so.