The U.S. spends far more, but the U.S. has far greater commitments.
Russia and China are "regional powers"; their goal is to dominate the areas in their geographic proximity. The U.S. has to dominate China in its region, Russia in its region, as well as the Middle East, the Americas, and everywhere else.
The U.S. has fought wars in Central Asia and in the Arabian Peninsula recently, as well as standing down an enemy in NE Asia (North Korea) and in Europe (Russia), facing a competitor in the western Pacific (China), and the Persian Gulf (Iran). There are deployments in over 100 nations, IIRC, and ongoing military operations all over, from the Philippines to Chad to Yemen to Somalia to many more. The U.S. Navy secures trade worldwide. And at the same time, the U.S. military must train and prepare for every contingency, from a Chinese invasion of Taiwan to revolution in Pakistan (endangering the nuclear weapons there) to a Russian invasion of the Baltics to a breakdown in order in Lebanon or Venezuala, to Iran restarting its nuclear weapons program ... China and Russia don't have those problems.
Here's an off-the-cuff summary of why: Since WWII, the U.S. has been the muscle behind the rules-based international order. Even domestically, under the rule of law, government needs force occasionally (e.g,, police) to maintain order and enforce laws. Internationally, there is no government; it's fundamentally anarchy. History has shown that the anarchy is very difficult to control and easily explodes into wars that destroy humans, cities, and set back civilization, and that's especially true in the nuclear age. Nothing is worse for humanity than war; WWI and WWII were major examples of that.
Post-WWII, the winning nations sought to prevent or minimize further wars by establishing a rules-based (i.e., legal) international order, centered around the United Nations and other institutions. The rules mean nothing unless someone can back it up, and post-WWII and even now, the U.S. is the only candidate with enough power to do so, military, cultural, and economic. The cost of the U.S. military is considered far less than the cost of anarchy, and in fact the U.S.'s preeminent position means that America has far greater say in international issues than any other nation.
The Trump administration is retreating from that responsibility, to a degree; we'll see how far they go and what the next administration does.
> The military has an efficiency problem, not a funding problem.
That idea has been around forever. Either it's not true, or it's intractable to a degree. Every large institution is wasteful, from Fortune 500 companies to large universities to the U.S. military. Unless you have a viable plan for reducing that waste, and can say what the savings will be, it seems wishful to say 'we'll just cut waste'.