That is literally their only choice. Ukrainians and Russians are dying by the tens of thousands. American soldiers would be better trained and better equipped than both sides, but they would also be dying in large numbers. We lost nearly 5000 thousand soldiers in Iraq, with complete and unchallenged air superiority, to disorganized religious radicals running around with 1950s era rifles and low-grade improvised explosives. We're all just vulnerable sacks of flesh and bone in the end.

And Americans would tolerate this for approximately 0 seconds. Consider how we feel about the hundreds of thousands of Iraqis that died or were killed during our invasion of Iraq (high end figures putting it in the millions), contrasted against the 5000 Americans who died. It's an entirely different sort of care when speaking of people you have no connection to as opposed to your neighbor or even your own son dying. When its other people, the real cost isn't truly felt.