It’s because they have been taught by Marxists, and don’t actually have a good understanding of world history, world politics or game theory. They think problems will sort themselves out in the absence of a hegemony that can broker the peace. Take Europe, if the United States hadn’t backed the EU, pressured Europe into giving up their colonies, and enforced a monopoly on force on the continent, then the culture would have stayed the way it was and been more nationalistic. There would have been arms races between competing great powers all over again, as there had been major continental wars ever 40 years for 3 centuries. This time it would have been cataclysmal. There would be no power of last resort to prop up the United Nations, and therefore there would be conflicts throughout the Balkans, mass genocide in the region of Yugoslavia, and a war between Turkey and Greece.
Or look at the Middle East. In the absence of the United States or Russia, Israel would be at war with every one of its neighbors who would all have to spend more on their self-defense instead of economic development. The epic Sunni-Shia struggle would have turned hot more often than it has. Egypt would be run by an aggressive socialist similar to Nasser. Pakistan would be at war with India. Jihadists would be everywhere.
If not for the United States, Latin America would have been colonized by European powers, and there would be monarchies and dictatorships. If Russia had won the dirty wars, Argentina and possibly Brazil would be where Venezuela or Cuba are right now. If the United States hadn’t supported Mexico and threatened to fight France, then Emperor Napoleon III would have held a monarchy in Mexico. Iraq would have still developed chemical weapons and used them against the Kurds, Syria would have still developed chemical weapons and used them against the people, and Iran would have developed nuclear weapons to fight the Sunnis. Everything would be more violent, and there would more destruction, poverty and frustration if the United States did not have an overwhelming active military at hot spots around the globe.
Most European politicians understand the vital role of the United States in preventing international anarchy and nipping arms races in the bud by amassing overwhelming force before they can begin. The governments want the United States to remain the dominant power on the European continent. They won’t ever try to build a sizable military when the United States can do it cheaper, and handle world conflicts better by tapping into economies of scale. Politicians merely attack the United States because it’s politically expedient, though it hurts the alliance. They know appealing to base nationalism and blaming a foreign power will score popularity points with a population which doesn’t need to think things through, and that understandably resents the feeling of having less say in world affairs.